Hi Sander. Marlin disables the extruder from working unless you heat the hot end up to 170C or higher. This is to avoid damaging things - usually you simply get a grinding of the filament but sometimes the bowden pops off.
So either heat up the print head to 180C or you can build a version of Marlin that has this safety feature disabled.
Have you heated up the nozzle? There's an interlock in the firmware that prevents the extruder turning when the nozzle is below a threshold temperature - iirc, it's 170ºC.
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