Can not Truncate the database because "secondaries has no log was added"

I got an error when trying to truncate the database: "Database can not shrink until all secondaries have moved past the point where the log was added". When I check the status of the secondary on the primary machine (Always On High Availability Dashboard), it has heath: good and Synchronization state: Synchronizing. But the status of database on secondary is NULL not ONLINE. When I check on the Always On High Availability of the Secondary machine, it's state is No data available on secondary replicas. What should I do and what is going on with the database..?
(I want to truncate because the free space is no more enough.)
Thank for reading. Pls help.

looks like there is some lag between the primary and secondary replica. can you check the last hardened LSN.
Also log_reuse_wait_desc in sys.databases.

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I have check and got the result like this

I have tried suspending, off and re-join the secondary replica databases on secondary machine (APEX-SQL2) but no thing change...

what is the log backup frequency?

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"Every day every 15 minutes".
In addition, my database is clustered and in HA state. And in theory, I could not change the full recovery mode to simple recovery mode.

No, you should not change the recovery model and if your log backups are running regularly as mentioned by you, there must be free space inside the Log files and shrinking them will just be overhead when it has grow back again.