While the message could be improved, you should only be seeing it if you try to import a v2 scheduler deck exported with scheduling into a v1 collection. If you’re sure you did not export it with v2 active, maybe you have an add-on that is interfering with things, as Anki will assume it’s v1 if the caller is monkey-patched.
I disabled all addons and tried again, same error. I even tried importing a deck that I just exported from the same v1 collection and got the error, so this seems to be a bug.
As for a less scary message I would suggest something like this:
The deck you are trying to import uses the v2 scheduler while your Anki collection still uses the v1 scheduler. In order to import the deck you must first upgrade your collection to the v2 scheduler as well. You can do so by clicking “Update” in the grey info box displayed in the main menu.