Thanks all for your replies. I usually don't use the online check-in either, but thought it would be helpful to speed up the process at the reception desk as I may be in a bit of a hurry when I arrive. I also wanted to use it to inform the hotel about my early arrival, hoping they would have a room available before the 3pm check in time. I will reach out to the hotel by email instead.

BTW The hotel is the Mercure in Bad Dürkheim. So definitely not a luxurious hotel.