Normally because the wrong License Username (usually full name) is being entered: https://www.mediamonkey.com/support/knowledge-base/purchasing-licensing-mediamonkey-for-windows/mediamonkey-license-key-usage/

I own licenses for both version 4 and 5. I want to put version 4 on a computer I'm going to use for an AV project but it tells me the code is invalid. Why is that?