If your machine is an SM9, you are in luck.  The platen removes very easily.  Simply unscrew the platen knobs and pull them out.  Draw the carriage to the right and the platen should lift out.

What I mean by the second questions is, I'm having trouble getting a good grip on the platen while trying to turn the knob.

Your typewriter is indeed an SM9, made circa 1974.  The right hand platen knob (obviously stuck) is a normal right hand thread. To give you some purchase to remove it, put the left knob back in and screw it up.  Operate the line space lever with your left hand and hold it across.  Then try to ondo the right knob.  The pawl being forced into the linespace ratchet should prevent the platen from turning.  Don't overdo this - you do not want to break anything.  A professional would use a special strap wrench which wraps round the platen itself.  You could also try to get a grip on the platen by wrapping a piece of emery cloth around it.  Operate the paper release and try to tease the emery cloth down the back of the platen (keyboard facing you)  Insert the emery cloth at either end, and not near where you think the label is stuck.

Helping each other is what this forum is all about - and comparing notes with others.  Now that your problem is solved, don't go away !  Stick around and see if you can maybe help someone else - or learn more !