Music transfer from laptop to iPhone blocked. ? reinstall iTunes?

laptop: mid 2010 macbook pro running high sierra 10.13.6

phone  iPhone 7+ with ios 14.4.2

both are working fine other than iTunes music issues

Core problem: cannot transfer any music from laptop(itunes) to iphone using USB to lightning cable(  this is music installed from my own cd’s)  I get the following message if I try to drag a song or album to iphone icon in itunes: “iPhone” cannot be synced because there is not enough free space to hold all of the selected items (additional 5 GB required). It's a 32 gig phone and it has over 12 gigs free. Also, while turingtest2 answered my first inquiry about this issue by guiding me through running mobile service updater, ( before that, the iphone was not appearing on the laptop when plugged in), anyway, his instructions caused the iphone to show up in itunes, but it turned out that the information about the iphone was incorrect and seemed to be leftover from when I had Apple Music. 

Background info: I think the basic problem stems from when I stopped using Apple Music. Is it wrong to think Apple Music must change things in iTunes or other systems, and that these changes remain even if you stop using the service?  Is there a way to get rid of all effects from Apple Music that might solve this problem?

turingtest2 suggested that if his operation did not solve the problem I might reinstall itunes on the laptop. I would need help/advice/instructions to do this since it looks like I would need to uninstall it first . I have an external hard drive where I could store the music files and it might be good to sort out unwanted music anyway.