I think it's the following problem: When you connected Garmin: Have you checked the "allow historical data access" during the connection? It's not toggled by default. Please reconnect and try again and try the historical import again.
https://runalyze.com/tool/garmin-import
As this is completely new from Garmin there is currently no information on the history import site where we check if you have toggled the checkbox during setup.