What happened?
The MMU doesn’t rely only on its sensors (SuperFINDA and the extruder filament sensor). It also keeps an internal record of whether the filament is loaded or unloaded. This helps the unit stay reliable even if the sensors are miscalibrated or damaged.
When the MMU records that filament is loaded into the extruder, a green light appears for that filament position on the unit.
If you try to load a different filament while the MMU still has a record that filament is already loaded, an error message will appear.
How to fix it?
First, hit Unload filament in the LCD menu. The filament will unload, and the Filament Loaded record will be cleared.
In case you have unloaded the filament by hand, the printer might not know about it. Next time, use the Unload Filament function from the menu.