Hi it may be that the account used by you sql server agent is a local account rather than a domain account and/or does not have enough privilages to connect to or update the monitor server

But you the following check list might help a little

Make sure that the SQL Server Agent service is running for the secondary server instance and that the restore job for this secondary database is enabled and is scheduled to run at the appropriate frequency.

The restore job on the secondary server might be failing. In this case, check the job history for the restore job to look for the cause.

The log shipping restore job, which runs on the secondary server instance, might not be able to connect to the monitor server instance to update the log_shipping_monitor_secondary table. This might be caused by an authentication problem between the monitor server instance and the secondary server instance.

The backup alert threshold might have an incorrect value. Ideally, this value is set to at least three times the frequency of the restore job. If you change the frequency of the restore job after log shipping is configured and functional, you must update the value of the backup alert threshold accordingly.

When the monitor server instance goes offline and then comes back online, the log_shipping_monitor_secondary table is not updated with the current values before the alert message job runs. Error 14421 can be raised when a restore job succeeds with, "Could not find a log backup file that could be applied to secondary database." When this occurs, the restore time is not updated. The cause of the error in this case might be an issue with the copy job.

To update the monitor tables with the latest data for the secondary database, run sp_refresh_log_shipping_monitor on the secondary server instance.

On the secondary or monitor server instance, the date or time is incorrect. This may also generate alert messages. Possibly the system date or time was modified on the one of them.