The value for the parameter @MirrorDirectory is not supported error with Ola Hallengren Backup script
The backup procedure does a number of validation checks against the path to ensure it is valid. One of them checks the edition of SQL Server to ensure it can perform mirrored backups.

Run the below code and if it does not return 3 then you don't have Enterprise Edition and cannot use Mirrored Backups:

SELECT SERVERPROPERTY('EngineEdition')


See: https://docs.microsoft.com/en-us/sql/relational-databases/backup-restore/mirrored-backup-media-sets-sql-server?view=sql-server-2017