So my sync stopped working for iOS devices in the Finder in Mac OS 10.15 Catalina. As soon as I made any change to the sync the apply button was greyed out.
Well I went to my /Library/Preferences/ Folder and removed the com.apple.syncserver.plist and the com.apple.finder.plist file (I am assuming it just one of these) but after removing them the apply button was no longer greyed out and I can sync my iOS devices again. You should then restart the finder.
•••
Update, so this fix is no longer helping me. I just can’t sync anymore. The sync button is OK until I make any changes, then I cannot sync.