Secondary standby server log shipping backup job error
It is clear the credential used for the copy job cannot be validated by the server that stores the T-Log backups or copied T-Log backups.

What account have you configured for log shipping copy job?

To access the destination disk, the admin there has to create an NT user and grant permissions. Ask for the help of your Windows administrator.

Verify that the account in use by SQL Server Agent has a login at the secondary server with sufficient permissions.

Make sure a login is created at Windows level with proper access to destination server.