You need to take a little off the sides of a new mortice blank as well in most cases or else they get stuck like yours
You will have to take the cover off and take the blank out, you did use a practice lock and not one in use didnt you?
If youve done the deed and its on a door in use

you will have to start trying to turn it back with a wire if non bs and if bs you probably are going to have to cut the blank off flush with the curtain and then take the lock out of the door,
If you have done it on a locked door, well just hope you havent

ask if so and we will have to continue instructing you after weve stopped laughing (no offence meant).