I found an issue in #NextCloud sync, and a fix, in case someone else is stuck...
If you're trying to add a sync pair and keep getting told it's already being synced even though it's not -
Go to the folder in question on your computer, reveal hidden files, and delete .nextcloudsync.log
That file is making NextCloud sync think it's currently synchronizing it via another folder pair.
I run in to this a lot on Linux Mobile.