Is this a new issue? Have you had the lens for awhile and taken the cap off previously?
If it is the type of cap that I have on all my RB lenses, they are locked into place by rotating the lens locking ring. I prefer those caps over any of the others for that feature. However, you obviously need to know that is the case and rotate the ring to unlock. If looking at the front of the lens you should be able to rotate the ring CCW or left to free the cap. It is highly advised not to use any undo force to move the ring, simply put, no tools applied.