I'm unfamiliar with the stickbike design internally - are you able to unship the rear chain, and try pedalling just the front chain? Of course you won't move, but if the clicking is evident you've eliminated the rear half. If it DOESN'T click then that really proves nothing, because this test is under no-load.
Check the mileage on your chains and on the teeth too - it could be the chain has worn and now has more slack. On a `bent that can add some significant droop to the longer runs, which could produce noise touching the inside of the frame ?
Try riding and coasting on a quiet smooth road; does the clicking stop while coasting?
Try backpedalling - does the clicking reappear?
Ask another rider to help with diagnosis - ride that same quiet road and see if they can get a "direction" on the click. As the rider, sound can seem to come from one area but be from somewhere else.
I see in some photos that the bike's main "beam" can be angled at the hub. If you change this angle does the clicking get more obvious or go away ?