And where can I learn this all?
And from my 2¢ below:
Why not to use it in actor blueprint?
- because it dramatically and often irreversibly complicates input handling
- because you may have 20 actors in the level and when you press Space, all 20 will jump… (you may want this but usually don’t)
- because input is (and, in most cases, should) be consumed when used
- because it’s not the actor’s role to handle input
- because there is a better place for it - Pawn or Player Controller - they are the ideal input instigators and processors
- because of order in which input is handled (check out the Input Processing Procedure in the link)
By default, input is disabled for regular actors:
How you came to know it when you were
learning?
- 40% digging through documentation
- 20% experimenting (by failing and figuring out what works & what does not)
- 10% helping others understand it better
- 10% digging up Forum posts and sifting Answer Hub
- 40% - stuff I still don’t know
That’s like 120%…, I know. Assuming here there are things that I don’t know that I don’t know yet. 
Place that node in the Player Controller / Pawn or even Level Blueprint and see if it works then.