Replying here because I ran into this issue, and while I believe Coqui’s is the most correct, there is another cause (completely unrelated) which will appear as a very similar problem: the in-viewport will begin to run, but the character will not spawn.
It can occur when the “Play” button has been set to “Simulate.” Fix: in the Play button’s right side dropdown, make sure the top option “Selected Viewport” is selected. 
(I can attest to the frustration of trying to troubleshoot gamemodes, blueprints, and pawns, before discovering that pressing “simulate” in that one blueprint… yep that had done it.)