This error is quite common and I have received it few times while I was working on a recent consultation project.
Cannot open database requested by the login. The login failed.
Login failed for user ‘NT AUTHORITY\NETWORK SERVICE’.
This error occurs when you have configured your application with IIS, and IIS goes to SQL Server and tries to login with credentials that do not have proper permissions. This error can also occur when replication or mirroring is set up.
If you search online, there are many different solutions provided to solve this error, and many of these solutions work fine. However, I will be going over a solution that works always and is very simple.
Go to SQL Server >> Security >> Logins and right click on NT AUTHORITY\NETWORK SERVICE and select Properties
In newly opened screen of Login Properties, go to the “User Mapping” tab. Then, on the “User Mapping” tab, select the desired database – especially the database for which this error message is displayed. On the lower screen, check the role db_owner. Click OK.
In almost all such cases, this should fix your problem.
Reference : Pinal Dave (https://blog.sqlauthority.com)