U.R.Flipper is upper right flipper, or in this case, the right side flipper.
This could be your flipper button controller, which is optical, and may need cleaning, or adjusting.
The Fliptronic II board controls 4 flippers. In the case of WCS, the upper flippers are the actual flippers, and the lowers are the 2 magnets (ball capture and magna save).
I would go into you switch tests and test the flipper buttons to see if they are reading properly.
Does the right flipper work?
It's also worth noting that the flipper buttons are on the matrix, so it's possible that there is a broken connection some under the playfield on either the x or y wire for the flippers. Case in point... I had an error on the trough, which is all optical, and actually controlled by a board under the playfield near the left flipper. However, the problem was not the optos, nor the board, but a broken wire on one of the striker targets by the left ramp. I learned a lot from that one, but mostly the fact that the cause may not be the error reported, especially when it's something on the matrix. I figured it out by doing the switch tests, testing every other switch in the same column in the matrix, then the same row. It immediately revealed the problem... A single broken wire, far from the ball trough.
For the flippers, everything is in the 9th column of the matrix in the tests. For the right flipper, it should light (from top to bottom) the U1, U2 and U6 (your offending switch), for the left, it should light U3, U4 and U8.
It looks to me as though your end of throw switch on the right flipper mech (U6) is your issue.