What happened?
MMU3 operates with 5 “tools” (materials - filament positions), labeled 1 - 5 on the unit. The desired filament position is selected by G-codes T[number]:
T0: Filament 1
T1: Filament 2
T2: Filament 3
T3: Filament 4
T4: Filament 5
Tx: Lets the user select a filament position from the LCD menu
T?: Legacy G-code for Tx
If you, for example, issue a T5 command to the printer, this error will be shown, because the tool range available for MMU3 is T0-T4 only.
How to fix it?
If you use a 3rd party software to control the MMU printer, make sure the number of extruders is set to 5; counting from T0 to T4.
In case you are using a 3rd party hardware solution allowing for more than 5 materials to be used, please use the corresponding 3rd party firmware for the device too.