Forerunner 255 Music installs updates at least once a day

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That is not by design. Did your device ever update to 18.23 successfully? You can check if you bring up your menu (long hold middle left button) and scroll UP to System > About. If your device is behind, I would recommend attempting to check for updates using Garmin Express. This would require a computer. If you have access, please follow the steps from Using Garmin Express to Update Your Watch's Software. 

Thanks. My watch is still running 15.19.

I've tried using Garmin Express (7.20, from an M1 Mac), but I get a transfer error. (See attached screenshot.)

I've also tried installing updates individually, but they all get the same error.

Someone somewhere on this forum mentioned they had failed updates, and it turned out it was because there was not enought free space on the watch. So maybe try to remove some music before updating?

Thank you for sharing the screenshot. 

Can you please follow steps 1-21 from Full Uninstall of Garmin Express/Garmin Express Fails to Uninstall, re-download Garmin Express, re-add your device and search for updates again?

Let me know if that works!

Thank you! That did the trick. I removed a couple of Spotify playlists and was able to update.

It appears to have been a lack-of-free-storage problem, as per the comment from Cooad, but thanks for your help.