<Please see attached file for image>
1. login to the DB environment where MSSQL 2016 is installed.
2. Login to DB via SSMS using windows authentication mode.
3. Right click on the localhost/machine name coming in the first line and select properties.(Properties panel will open)
4. select the security tab from the left pane of the properties panel.
5. Server authentication mode to SQL server and windows authentication mode (That is the second check box).click ok if it ask to restart.
6. after this you have to restart the MSSQL service so for that go the services by and look for MSSQL restart the service click of if it ask for more services to start along with MSSQL. <Please see attached file for image>
In Object Explorer, expand Security, expand Logins, right-click sa, and then click Properties.
On the General page, you might have to create and confirm a password for the login.
On the Status page, in the Login section, click Enabled, and then click OK.