If your MMU makes a loud clicking noise as the selector changes position, and if you see that the idler barrel was unable to rotate into the correct position, it is likely that your MMU idler is over-tightened. Loosen the two screws (one on each side of the MMU unit) a little bit and try again.
To find the perfect tension, tighten the screw about 0.1 mm to 0.4 mm under the edge of the hole on both sides of the idler body. All 5 filaments have to be loaded to the hobbed pulleys position. Then move with the selector to the far right (5th filament position) and back and observe. If you hear clicking, loosen the screws a bit and try to move with the selector again until there is no more clicking.
If the idler still can’t rotate freely, you can open the idler completely and verify that nothing's blocking it.
Check if the idler is centered and if the edges of the idler don’t grind the idler body. See if the idler is correctly tightened by the two M3 screws against the flat part of the motor shaft.