Have you installed MySQL Driver on the server? If not you need to use following URL and install MySQL Driver on Tableau Server.
Sometimes, if you keep the port blank, it should work, I had that issue in the past.
I have installed MySQL Driver on the machine where TS is installed (it was installed properly as I can connect to the MySQL DB using Tableau Desktop, from the same machine as TS). If this have any significance, we have MySQL 5.0.51a-3ubuntu5.1 and the TS is on a Windows Server 2016. The driver installed is MySQL ODBC 5.1.
Also, the trick with blank port didn't help...