I just ran into this same issue. As others have stated, creating the Map Area on the device worked, but I looked a bit further and found what the issue was in my scenario and can now create Map Area's online and successfully edit on the device.
I tried to edit my layer using the Web Map in AGOL, but received an error message stating "Feature creation is disabled for all layers."
Searching for that error led me to this support article:
It turned out that my point layer had z-values enabled, but the option to "Apply default to features with z-value" was not selected in Pro when publishing the layer.
Long story short - you can either remove z-values from the point layers OR enable that option in Pro when publishing the layer.
I returned to Pro and re-published the layer with that option enabled. It did NOT show "Enable Z Defaults" as "true" at the REST endpoint after the initial re-publish, so I re-published it again and the value updated at the REST endpoint.
I then returned to the Web Map and editing was working there, so I created a new Map Area online, downloaded it on the device, and voila - IT WORKS! I can now edit on the device using Map Areas created in the Field Maps Web environment.
Not sure if this will work for your scenario, but it is worth a shot!