I think the tournament will not take place at all. It is foreseeable that the new highly contagious virus mutations will spread quickly. Even in Japan, more and more votes are being voiced to cancel the Olympic Games. If there was a bet on whether the Euro Championship would take place, I would currently bet on no.
Question about Olympic Games is still open, but I'm confident that Euro 2020 will happen. All national leagues are in play, Champions league too, even tournanents like Nations league are being played. I don't see how Euro is different from it.
And maybe there will be spectators in stadiums. As we saw last summer, spread of virus wasn't such big as we have now. Also, in June, some part of population will be vaccinated already.
So, I don't see how Euro can be cancelled.