exported MP3s not working on device

I’m using Windows 10 21H2 and Audacity 3.1.3.

I’m hoping someone can help me. I’m not very proficient with Audacity and have always only used it for minor things such as cutting long silences at the end of a song MP3. I have a stereo in my old truck that I play MP3s on using a flashdrive. My stereo has no problem reading MP3s that I’ve either downloaded or ripped from a CD. It also plays MP3s that I edited several years ago with Audacity v2.something with the Lame plugin. However, I edited a few song MP3s today using Audacity 3.1.3 but my truck stereo doesn’t recognize them. I’ve tried setting the project rate at 44100Hz as I read on your FAQ but this doesn’t fix the problem. Does someone have any other suggestions as to what I might try next? I appreciate any help.

Check that the flash drive is working OK by putting a “known to be good” MP3 on it, and check that it plays correctly.

If that’s OK, then make two MP3 exports from the same Audacity project, one with your current settings and one with the “Bit rate mode” set to “Constant” (see: https://manual.audacityteam.org/man/mp3_export_options.html)
Name them appropriately so that you know which is which.
Put them both onto the flash drive and check if one or both of them play.