It looks like, for whatever reason, the Tableau Server is unable to connect to the SQL Server. This is not a user/password problem as that would give another error. My best guess would be that there is a firewall blocking the connection and you'd need to have your network folks open up the port to allow this traffic. You'll also need to ensure that named pipes connections are allowed on the SQL Server. See the following for details on how to do that: https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/enable-or-disable-a-server-network-protocol?view=sql-server-2017#SSMSProcedure
This looks like firewall issue at SQL Server Hosting machine, Plese contact your IT Team to allow required firewall to Tableau Server.