your coordinates (vector(-20000,12224,16640)) are wrong. the coordinates of a layer or view entity are relative to the camera! so your arrow must be very very far away from the camera so that its too small to see.

try something like vector (100, 0, 50)

the first value (x) is the distance to the camera,
the second one (y) is left/right
and the third one (z) is up/down

hope that helps!
- SchokoKeks