What happened?
Other than the sensors (SuperFINDA and Extruder filament sensor), the MMU also has a record of filament being loaded or unloaded. This way, the unit can stay reliable even in case the sensors are not properly calibrated or are damaged.
If there is a record of filament being loaded into the extruder, there is a green light for the corresponding filament position on the MMU unit. If you attempt to load another filament while there is a record that the filament is already loaded, this error will be shown.
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.