messages not syncing

Open messages and its preferences in Mac , initially the box of enable this account was checked the sync button appears as normal .

A pop up appears , you clicked over disable this device .

After unchecking the box , the sync icon is greyed .

Once again click on enable messages iCloud , the messages starts uploading .

See the articles Change iMessage preferences in Messages on Mac - Apple Support

Set up Messages on iPhone - Apple Support (IN)

Did you followed this article Set up Messages on iPhone - Apple Support

Set up Messages on Mac - Apple Support

Access your messages on all your Apple devices - Apple Support

This is old information and is a good idea but is not solving the problem for me.