Hi,
It is likely the underlying webspace for your existing app service plan doesn't support mv sizes. What you can do is create a new web app with new app service plan in different resource group and specify Premium v3 P2mv3 during creation. That way it will be placed in webspace that supports it.
Once you have the new web app you can re-configure it to match your existing one, deploy files to it, etc. After everything is correct with your new web app you can delete old web app and its app service plan to avoid being charged for both on ongoing basis.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP