I’ve had a similar problem syncing mp4 files to my phone before, even when the files played fine if I copied them over manually. Sometimes, it turned out the video profile or encoding settings made certain apps or devices extra picky, and the sync process just refused some files without much explanation. Usually, if I re-exported the videos using a different tool or combined a few mp4s into a new one, they’d sync and play without a hitch.

[quote=Ludek post_id=499939 time=1659695991 user_id=7319]
So what is the error message when you right-click the file > Send to > S21 ?

Does this happen also with auto-conversion disabled: https://www.mediamonkey.com/wiki/WebHelp:Exporting_Tracks/5.0#Auto-Conversion ?
i.e. when you uncheck all checkboxes in Sync Profile > Auto-conversion and click [Apply] ?
[/quote]

Haven't tried it yet but everything is set to not convert anything. my phone is good enough to play common files, I would think.

So what is the error message when you right-click the file > Send to > S21 ?

Does this happen also with auto-conversion disabled: https://www.mediamonkey.com/wiki/WebHelp:Exporting_Tracks/5.0#Auto-Conversion ?
i.e. when you uncheck all checkboxes in Sync Profile > Auto-conversion and click [Apply] ?

[quote=MiPi post_id=499877 time=1659528311 user_id=27459]
Which Build (Help > About) and phone device are you using?
[/quote]
A Galaxy S 21+
using build 5.0.3.2627.

Which Build (Help > About) and phone device are you using?

General
Complete name                            : C:\Users\Eric\Videos\Music Videos\Country\Shania Twain Live In Chicago 12 - No One Needs To Know.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 98.8 MiB
Duration                                 : 3 min 16 s
Overall bit rate                         : 4 206 kb/s
Season                                   : 0
Part                                     : 0
ContentType                              : Movie
Encoded date                             : UTC 2021-05-13 06:31:08
Tagged date                              : UTC 2021-05-13 06:31:08
Writing application                      : Lavf58.12.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L5.1
Format settings                          : CABAC / 1 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 1 frame
Format settings, GOP                     : M=1, N=30
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 3 min 16 s
Bit rate                                 : 4 003 kb/s
Width                                    : 720 pixels
Height                                   : 540 pixels
Display aspect ratio                     : 4:3
Frame rate mode                          : Constant
Frame rate                               : 30.000 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.343
Stream size                              : 94.0 MiB (95%)
Language                                 : English
Encoded date                             : UTC 2021-05-13 06:31:08
Tagged date                              : UTC 2021-05-13 06:31:08
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 3 min 16 s
Bit rate mode                            : Constant
Bit rate                                 : 196 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 4.60 MiB (5%)
Language                                 : English
Default                                  : Yes
Alternate group                          : 1
Encoded date                             : UTC 2021-05-13 06:31:08
Tagged date                              : UTC 2021-05-13 06:31:08

I get error messages on music videos in mp4 format that won't sync to my phone. I can't figure out why it won't sync. i'm not sure what isn't compatible. they have played when i manually copied to my tablet and phone before. But i wanted to sync music video playlists from Media Monkey. here's the MediaInfo data:
[code]General
Complete name : C:\Users\Eric\Videos\Music Videos\Country\Shania Twain Live In Chicago 12 - No One Needs To Know.mp4
Format : MPEG-4
Format profile : Base Media
Codec ID : isom (isom/iso2/avc1/mp41)
File size : 98.8 MiB
Duration : 3 min 16 s
Overall bit rate : 4 206 kb/s
Season : 0
Part : 0
ContentType : Movie
Encoded date : UTC 2021-05-13 06:31:08
Tagged date : UTC 2021-05-13 06:31:08
Writing application : Lavf58.12.100

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L5.1
Format settings : CABAC / 1 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 1 frame
Format settings, GOP : M=1, N=30
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 3 min 16 s
Bit rate : 4 003 kb/s
Width : 720 pixels
Height : 540 pixels
Display aspect ratio : 4:3
Frame rate mode : Constant
Frame rate : 30.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.343
Stream size : 94.0 MiB (95%)
Language : English
Encoded date : UTC 2021-05-13 06:31:08
Tagged date : UTC 2021-05-13 06:31:08
Color range : Limited
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709
Codec configuration box : avcC

Audio
ID : 2
Format : AAC LC
Format/Info : Advanced Audio Codec Low Complexity
Codec ID : mp4a-40-2
Duration : 3 min 16 s
Bit rate mode : Constant
Bit rate : 196 kb/s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 48.0 kHz
Frame rate : 46.875 FPS (1024 SPF)
Compression mode : Lossy
Stream size : 4.60 MiB (5%)
Language : English
Default : Yes
Alternate group : 1
Encoded date : UTC 2021-05-13 06:31:08
Tagged date : UTC 2021-05-13 06:31:08[/code]