If you press on the door part does it flex downward? Basically seeing if the door is free just caught on the edge.
If it is then what @oldturkey03 suggested to carefully pry along the latching edge. Removing the bottom cover screws won't get you to the hidden one within the battery chamber. So consider that as you'll then need a new bottom cover and door vs just the door if you cut it out [https://www.ebay.com/itm/293621354147|Battery Door For Canon EOS 6D Digital Camera] & [https://www.ebay.com/itm/253861186904|Bottom Cover Base Cover Plate Assembly For Canon EOS 6D Digital Camera]
If there is no flex then it's possible something sticky got in the gap.