This sounds like something I went through a little while ago… Computers were synced once before, but not doing so lately… The gist I took to reenable sync to not sit there dumbly, was:
#1. reboot the computer
#2. On the source computer, where I get the sync code to copy, check the sync code hasn’t changed. In my case it did, so recopy the sync code.
#3. On the computer I want to sync, put in new sync code.
Sync’ing then “worked”… I put it in quotes because while syncing took place, it treated everything as never been synced and ended up with double bookmark entries in both computers…
So… my suggestion to you before getting sync to work is to first get a copy of your source computer’s bookmarks manually by going into bookmark manager and exporting bookmarks via Hamburger (3 vertical dots) menu > Export as an html file. Then you can go about resetting sync if you want, but honestly with a hard copy of your book marks, which can the be imported through bookmark manager, I wouldn’t bother with sync.
Edit: Note I’m not sure rebooting the computer was actually necessary…