Can not give NT SERVICE\MSSQLSERVER permissions on network drive
NT SERVICE\MSSQLSERVER is a local service account - therefore it does not have access to network resources (like file shares)
Change to either a Network service account or a domain account using SQL Server Configuration Manager
The different service account types are described here :-
https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/configure-windows-service-accounts-and-permissions?view=sql-server-ver15