Can't Create disposable VM?

I am trying to create disposable vm for fedora 34 .

according to the Doc
Disposable customization | Qubes OS

can’t run this commnad

qvm-prefs fedora-34-dvm template_for_dispvms True

what is wrong here?

should I create disp sys-net and sys-firewall also after installing my disposable template ?

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.
Also "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

My new template like this
Domain: custom-disposable-template
It isn’t disposable template …
I have followed this doc
from this step < Creating a new disposable template>