Please provide more information.
Are you synchronizing folders in the folder panel (to import new photos)?
Or are you synchronizing photos to send to the Adobe Cloud?
An error messge 'File not found' would indicate the files do not exist, or have been moved or renamed.
Post a screen-clip of the error message and the grid view of photos in your catalog.
Regards. My System: Lr-Classic 12.2 Photoshop 24.2, ACR 15.2, Lightroom 6.2, Lr-iOS 8.0.8, Bridge 13.0.2, Windows-11.