Error: 18452 Login failed for user ‘(null)’. The user is not associated with a trusted SQL Server connection.

I had an error that I believe a lot of new user for SQL server may face during create a new Database.

Below is the resolution:

Change the Authentication Mode of the SQL server from “Windows Authentication Mode (Windows Authentication)”
to “Mixed Mode (Windows Authentication and SQL Server Authentication)”.

Run following script in SQL Analyzer to change the authentication

LOGIN sa ENABLE
GO
ALTER LOGIN sa WITH PASSWORD = ‘’
GO

OR

In Object Explorer, expand Security, expand Logins, right-click sa, and then click Properties. On the General page, you may have to create and confirm a password for the sa login. On the Status page, in the Login section, click Enabled, and then click OK.