Why is the game telling me I cant teleport because I'm observing another kingdom?

If you are observing another kingdom, your city is not located there. To go to that other kingdom, you must migrate, not teleport. Your teleport is limited to your own kingdom, and your troops must be housed inside a building or structure in order to teleport. Note, the building or structure does not have to be yours. It can be a flag, a pass, or another alliance city, but it must be in the kingdom where you are teleporting. You can't teleport to or from the lost kingdom if your troops are outside your city.