So I've tried to make an Event in which the player recieves an Item, but when I test it, the Pokeball's not here. Also, the game asks me to register a Trainer when it's already registered. Can anybody help me?
Showing the build of your event wouldn't be bad. Otherwise it's anyone's guess. Also are you sure, the trainer is registered properly? Sometimes a little spelling mistake can slip in that you don't see immediately. You could also try to force the compiling by pressing CTRL while the game starts.
Diverscope answered for the trainer, it's probably a mispelling indeed.
For the pokeball are you sure your event doesn't have 2 pages with the second page blank? If it's the case then maybe you didn't tick the "if local switch A is on"
by default the game will read the last available page of an event so if you forget this, it will ignore the first page where it should give you an item and will launch the second page (if you interact with the invisible event) and won't do a thing because the event is empty.