In Catan, is it possible to steal from yourself with the robber?
No, you may not.
According to the official rules on the website:

(1) You must move the robber immediately to the number token of any
other terrain hex or to the desert hex.
(2) Then you steal 1 (random)
Resource Card from an opponent who has a settlement or city adjacent
to the target terrain hex. The player who is robbed holds his Resource
Card hand face down. You then take 1 at random. If the target hex is
adjacent to 2 or more players’ settlements or cities, you choose which
one you want to rob.

One could argue the second part &quot;if ...adjacent to 2 players'&quot;, but I find that interpretation completely mis-aligns with the unambiguous previous rule that you do not and would not rob yourself.