Sometimes it happens when you did everything correct like Installation with mix mode, created new user assign credentials etc but still not able to login with the username and password credential.
You get “Unable to Login – Microsoft SQL Server Error: 18456”.
To resolve this error you need to do following steps
1) Login with Windows Authentication
2) Open server properties using right click
3) Go on security TAB
4) Check SQL Server And Windows Authentication mode Checkbox
Now you can login with user credentials.
Thanks & Best Regards,