Every time I setup a new SQL Server I always forget one or more of the steps below to allow remote connection from another workstation via SSMS.
1. SQL Server Properties: check “Allow remote connections to this server”
2. SQL Server Configuration Manager: Enable TCP/IP
3. Windows Firewall – Add Inbound Rule for the SQL Server port.
For detailed steps check out: Resolving could not open a connection to SQL Server errors By: Jugal Shah