Adjust command line in adding a MST is blocked

I had a clean run first with a MSI as the MST was not ready when I started.

So, plain installation is running fine and the MST is ready.

I wanted to add this MST to the CommandLine in the Package using the <Edit> button next to the commandline-entry. But the <+ Add> is greyed-out in the MSI-wizard.  Help (*) text: "Unable to add new transform due to a managed package not being supplied"

(*) Did that help?  Noop.

Anyone in plain English?

ps, added it to the commandline manually adn that worked, but never the less..)