Cannot open backup device

SQL Server throws an error message when restoring a database backup file:
Cannot open backup device ' \\xxx\yyy.bak '. Operating system error 5 (Access is denied.).

The reason for the error is that SQL Server requires access to a resource other than the instance of the Bak file, but SQL Server services does not have sufficient permissions to throw an error message that access is denied ("Access is denied"). The MS SQL Server Services account must be given sufficient permissions.

Press the key: Windows+r, enter services.msc, open Services, locate Service:sql Server (MSSQLSERVER), open its properties form, switch to log on tab.

Modify the default NT service\mssqlserver to the Windows domain account and give the appropriate permissions.

Cannot open backup device