tabcmd is a Server tool. You can't run it against a Desktop file.
You're going to need to do it against Server, though, and not Desktop
We are looking for a way to avoid having to publish ever workbook to the Tableau Server because our site doesn't have enough space and I don't think we will be successful requesting more space be added. For example a single data source file is over 10GB.
I don't believe there is anything in the tableau server license conditions that limits the size of your vizzes.
The size limit is set by our Tableau Server Admin on our Tableau Server Site.
The admin is trying to avoid having to add additional storage to the server.