Hello!
When I switch from Mainmenu level by in game “Start” button to the battleground level i can’t move the tank camera. But when I open Battleground level in editor separately, everything work. Any ideas how to fix it? 
For me, I used a OnClicked event from the start button.
OnClicked(StartButton) -> Set Visibility (self to hidden) -> Open Level (name of level)
Just so you know that any physics actors you have in the level will act poorly as they may clip with the terrain and collide abnormally.
The fixes for that are described here:
https://answers.unrealengine.com/questions/530234/i-have-weird-physics-when-i-load-a-level-from-blue.html
(FYI the accepted answer added SetSimulatePhysics in Tank_BP’s Event Graph).
EDIT: After watching the next lecture I got the same issue as you. Remember to set input mode to game only after loading the level.
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Thank you very much! Problem solved 