Lightroom Classic sync stuck

Is the Sync Feature Enabled in LR Classic?

Click the triangle in the Identity Plate and make sure Sync isn't Paused.

File Sync Links that may help... all the links I have, since I don't know the cause of your specific problem
-FAQ https://community.adobe.com/t5/Get-Started/Helpful-Links-FAQ-Where-can-I-find-links-about-managing-and/td-p/7570118
-http://helpx.adobe.com/creative-cloud/help/sync-settings.html
-http://helpx.adobe.com/creative-cloud/kb/arent-my-files-syncing.html
-sync and email link https://community.adobe.com/t5/Get-Started/Email-link-of-file-in-personal-CC-folder/td-p/5944719
-an overview of assets https://assets.adobe.com/files
-File sharing https://community.adobe.com/t5/Get-Started/Does-Creative-Cloud-support-sharing-videos/td-p/7113392