A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Hi David,
Sorry for the inconvenience, but unfortunately I cannot repro the issue you met on my test SharePoint site, and the sync icon display normally on the ribbon.
First, did you sync files in this site before? And is there a Sync icon before?
If yes, I think your local OneDrive sync client would sync them automatically once launched, you could open it to solve the current problem.
If no, I think the site may be a project site or has using Project sites template.(Or perhaps other classic templates), which won't show a Sync icon on the ribbon.
Additionally, some issues may only affect modern experience but not classic one, so I suggest you go to the document library to switch to classic mode first.
If there's no button to do so directly on the site, you can then go to Settings(the gear icon in the top-right corner)->Library settings –> Advanced settings –>Change the option in the list experience from New to Classic experience as shown in the image below.
By doing so you’re switching back to previous document library experience. And then please go back to see if there's a sync option.
If the problem remains, please feel free to post back, and we need a screenshot of the document library page( please remove PII)
Best Regards,
Oliver