Problem:
Some databases are skipped during the backup of a secondary availability group replica, stating issues with the synchronization state.
Cause:
Only databases in "synchronized" state can be protected when backing up secondary replicas.
Solution:
Usually, this is a temporary issue when a database is newly added to an availability group and has not yet been fully replicated. Re-try the backup later. If the problem persists or occurs repeatedly, ask your SQL Server admin to check the database replication of the availability group.