No permissions to the certificate file when trying to restore a DB from a third party
It seems you have two SQL Server instances installed. LEVVEL and a default instance. Which one are we talking about? I.e., which one are you connected to?

Anyhow, both services uses a virtual service account, so privileges assigned to NETWORK SERVICE is irrelevant.

For the default instance, you should assign privileges to NT Service\MSSQLSERVER.

The the LEVVEL, you should assign privileges to NT Service\MSSQL$LEVVEL.

When you assign privileges, you need to type the above account name.