Check that “Auto possess AI” on the pawn class you’re spawning is set to one of the options that include “spawned”.
By default this setting is set to “placed”, which means it only gets an AIController automatically if placed into a level. Changing it to “spawned” automatically spawns an AI controller when the actor is spawned via blueprints or C++.
Alternatively you can manually spawn an AI controller, or use the Spawn AI node.