I have the same problem... We upgraded from Desktop 2017 to Desktop Pro 2020 and now the excel import is failing with the same error... Unit of Measure is currently turned off and the mapping you selected was crated when UOM was turned o. Create a new mapping or edit the current one and save it.

This is insane.

You can't turn UOM on since it's grayed out and we don't even need UOM. 

You can remove all of the mappings and recreate them and it still doesn't work.

Support has no clue. Just robots with no knowledge.

This is very frustrating.