It seems, the MMU unit has either an old or an incompatible firmware version.
The printer will show you an expected firmware version for the MMU unit.
For example, this error is shown when using the old MMU2S firmware 1.0.6 or below in combination with the newer printer’s firmware such as 3.13 or up on MK3S+.
Make sure that are using the correct up-to-date firmware for both the printer and the MMU unit: Firmware download.
If you are not sure which version you have, find the order number from the MMU unit, or take pictures of it, and send it to our Kundendienst.