Okay, so it sounds as though you do need the music in MP3 format. But as I stated, I've converted a song from AAC to MP3 yesterday, using iTunes (in order to check that what I am saying is correct), so I stand by what I said previously - the conversion process does work.
So now is the time to work out exactly what the issue is. Perhaps your files are protected (when did you purchase them?). Or - here's a thing, perhaps they have been converted but you just haven't found the conversions. Let's see what we can do:
- Error message: error message 42017 may indicate that the file is corrupted and I suppose that even if it plays correctly in iTunes (have you checked?), it's possible that it won't convert. The solution is to delete the song and download it again from your account purchases (I'd make a backup of the existing file first though)
- Protected Files; up until mid-2009 music sold by the iTunes Store was protected from copying by something called DRM. To check whether your songs are subject to DRM:
- consider when you purchased them. If they're recent then they're are unlikely to be protected
- examine the file's properties (in your iTunes Library). Select a problem song and click on Edit/Song Info/File>kind. Is it a Purchased AAC audio file or a Protected AAC audio file? Note the difference. Study the two songs pictured below, which I purchased from the iTunes Store (well, in fact they were free gifts, but the principal is the same):

- note that the purchase dates, in 2009, are only two months apart (UK date format - DDMMYYYY), which suggests that by May 2009, Apple was no longer subjecting music to DRM
- the expected error message if a file cannot be converted is as shown in the following screenshot:

If you saw no error message, that suggests that the conversion was completed, which leads onto the next stage.
- Converted but not found: a conversion creates a new file, in addition to the existing one (yes, this means that you now have two copies of the specific song - the original and the conversion). But dependant upon how you look at your Library, you may not see the newly converted files. Target one of your failed conversions: look under the Artist name or Album title. Can you see two copies of the converted file? Study the screenshot below:

- notice that I have four copies of the Lisa Hannigan song and that the top two were added today - as conversions (27/12/2023 is 27th December 2023). The third one is a conversion made in 2021, possibly for the same reason as today, while the bottom one is the original
Ethelrude wrote:
Rebuying the songs elsewhere means I wasted my money and that’s on Apple.
I'm not sure the law would agree with you, since the songs play as advertised (providing you have confirmed that they play, as mentioned above) and do what they are intended to do.
But if you are left with no other option (which I doubt), then chalk it up to experience and part of life's ups and downs. Certainly in the future, buy MP3s from somewhere else.