I had/have this issue too. Ridiculous.
But then, weirdly, after toggling all the switches in settings (find my device / location), AND opening Maps app (did this have anything to do with it?)... then going back to the website which still had "location disabled", THIS time, it was able to be enabled in the find-my-device webpage https://account.microsoft.com/devices/find-my-device
So... weird