In order to take measurements, the circuit between positive and negative poles of the multimeter must be closed.
You connected them to two devices that are not connected in any way, so there is no current flowing through the circuit and measurements are impossible.
Above is the answer to your direct question.
But in general, comparing voltages in two unrelated circuits is incorrect at all. Voltage is a relative value, it is always measured relative to some point, in electronics it is usually a GND (ground) wire.
Measuring voltage between two circuits that do not have a common ground is pointless
This is the schematic. Would a possible solution be to power the arduino with the same power supply instead of the USB form my laptop. That way they'll have the same GND?
Edit: thanks for the quick response btw
Edit 2: I connected my arduino to the same power supply and everything works
You appear to be looking to make a Low-side transistor switch, with an NPN transistor, with the power source at the base of NPN transistor being a different power supply from the load. But you have not explained this properly in your first post. Please provide further details.
If designing such a thing, you need to have the grounds of both power supply's connected together. More details on different types of transistor switches here:
