For printing with MMU, it is fundamental that all parts move freely and correctly. In case the idler is unable to move properly, the following message is shown: "The selector cannot move properly. Check for anything blocking its movement. Check the wiring is correct".
The error message indicates that the MMU selector isn't moving properly. Check for any debris or residue that might be blocking the selector movement, and follow this guide to check for other possible causes for MMU selector unable to move freely.