How do I get fast forward & rewinds to work on my Teac W450-R dual cassette deck..?

Check the belts, inside the unit you should see a rubber belt which is what pulls the tape fast forward or rewind. Some belts with time get loose and slip, they can also break and fall off which will not let you fast forward or rewind.
Open your cassette player and look where the plastic pulleys are located and check for the belts, they might not be broken at all but very loose and slip on the plastic pulley.