Cannot open database requested by the login. The login failed

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

1. Go to SQL Server > Security > Logins > right click on 'Your user error name' > Properties

In opened screen of Login Properties, go to the “User Mapping” tab.

2. Select map checkbox appropriate to your database, And change user dropdown to sa (if necessary)

3. Then change user membership to dbo.owner

Then click on OK

In almost all such cases, this should fix your problem.