Your terming is not quite clear, but the command you quoted serves to
prepare already created appVM named
fedora-34-dvm based on fedora-34 template, to become disposable template. And, fedora-34-dvm is by default created as disposable template, so it is not clear what are you trying to achieve running this command.
"can’t run this command" doesn’t look like properly used term. Anything can be run, but the outputs of the commands are different, meaning when you ran this command in the terminal what was the output?
If you are trying to create another disposable template, then you could follow this guide
… but existing disposable templates are sufficient for the start.
Although advisable, it’s totally up to you, and if you decide to do so you could follow this guide, and in case you’d like to create custom sys’s. you could follow this guide