i am having issues completing the installation of DNN 9.0.1. i did follow chris's video on DNN 9 installation. where i have a problem is in establishing a database connection. Below is the error i get when i click on continue button. i have been on this for two days now any help will be appreciated.

ERROR:Index #: 0
Source: .Net SqlClient Data Provider
Class: 20
Number: 87
Message:

Under Server Name i wrote : .\MSSQLServer

Under security: i checked USER DEFINED

then added by database username and password where appropriate.

i also did did checked : DATABASE OWNER

please help me.