how to convert AVI to MP4 with QuickTime on a Mac?

This can be a really complicated topic. It could be that all you need to do is repackage the audio and video tracks into a MP4 wrapper. Try Subler.

If you want to take the simplest route you can convert the files with the free Handbrake. It may unnecessarily convert some of the tracks with inherent quality loss but unless you want to learn more about digital movies it is the easiest way.

AVI is a fairly ancient Windows format and it is rare to encounter it these days.

Quicktime has never been able to handle AVI natively. Another approach would be to not use Quicktime but use an app such as the free VLC which can play just about anything. Since what you want to do is not possible then you will either have to download something that will play it, or you will have to just not watch the video..

View it in IINA.

https://iina.io/