When I try to record and I click on the arrow next to the Arm button, all I see is channel 1, channel 2 and so on

In Mixcraft, if you go to select the recording source on a track by clicking on the arrow next to an Arm button, and all you see is "channel 1, channel 2, channel 3 and so on," that means you are on an Instrument track, which is for MIDI only.

Instrument tracks have a little keyboard button on the left.

Find a track without the keyboard button and then you"ll be able to select "microphone" or "line in" or "USB audio codec" or whatever.