NT SERVICE\MSSQLSERVER cannot be found to add as user for file permissions
What else has to be done to make this appear as a user account?

Nothing.  And it doesn't matter what your service account is.  The SQL Server service always has privileges assigned to the per-Service SID &quot;NT Service\MSSQLSERVER&quot;.
If the files are on the SQL Server, just add permissions for this account:

And if the files are on a remote share, give the permissions to the machine account instead, eg &lt;YourDomain&gt;\,&lt;YourServer&gt;$.