in one of our document library the sync is disabled when we tried to configure sync to a folder. the sync is working on other document libraries. The "Allow items from this site to be downloaded to offline clients?" is set to yes . but still it is disabled.
Ensure that Offline Client availability is set to yes in that doc lib as below. If its set to No, click on Yes and then OK
Library settings -> Advanced Settings -> Offline Client Availability
In the "New/Modern" experience, you can select it as below:
Also, if its a
picture library then it will be
disabled as its supported.
Reference - Sync button grayed out