The other person has not printed postage yet. (At least as far as I can see they haven't.) But there's no button or anything I can click on to cancel my request on my books I've requested tab. Thanks!!!

If the other person has accepted it and printed the label, then they need to cancel it for you.  You can cancel up until they print the label.  Once the label is printed, only the sender can cancel. (This is a built in protection to stop folks from canceling when the other member has already left for the post office.)

If you are asking them to cancel, be sure to PM from the book transaction page.

Be prepared for them to say no though.  If it's a heavily posted book that they've waited a long time to get to the top of FIFO and get requested.  They may not be inclined to cancel and have to wait all that time over again for another request for the book.

Yeah, in that situation they are not obligated to cancel as they can be disadvantaged by doing so.