I cannot convert an old Mathcad worksheet to a Mathcad Prime worksheet. Mathcad says "No license."

As far as I know you are using Prime 9

Do you get the "no license" message when you try to start up Prime?

If you are able to fire up Prime, are you sure its the full version or are you seeing some locks in he menu indicating that premium functions are not available (Express version).

When you installed Prime, did you make a check mark to install the converter as well (its not checked by default). But if you forgot to install the converter, you would not get a "no license" message but the menu for conversion would simply be greyed out.

What do you do when the error message pops up? Can you post a screen shot?

It looks like something you should discuss with PTC support.