I just can’t remember how many times I’ve seen this same error popping out to me when I was trying to connect to a new SQL server. And every time when I see this I had to dig into my mind figuring out what to do to solve this. So it’s time to jog down the notes for the next time. My brain just sucks when trying to remember stuff like this.
1. Enable TCP/IP protocols from SQL Server Configuration Manager. Make sure to restart the SQL server after the change.
2. Turn on Port 1433, or whatever the number you set on the server, on Firewall.
3. If the target server is a named instance, such as server\instance, the service SQL Server Browse needs to start. And a UDP port 1434 needs to be turned on on Firewall.
If still no luck, go through this checklist to see what’s missed.