Assuming you are trying to erase objects on the game board, make sure you have selected the arrow tool in the upper left, and that you're on the right layer. E.g.: I tried and failed several times to erase a drawing I made on a map or a token, only to realize I draw it on the GM layer, and I was trying to select it on the objects layer. Drawings can also be erased en masse with the "clear drawings" option on the draw menu.
