Tournaments score sheet — opening


Nothing!
From FIDE Rules
8.1.4 The scoresheet shall be used only for recording the moves, the times of the clocks, offers of a draw, matters relating to a claim and other relevant data.

That field is optional.
If you know the opening, you have every right to enter it. If you don't know it is not obligatory.
For example, if I am Black, and the game goes 1.e4 e6 2.d4 d5, I will write in French Defense, but if it goes 1.h4 e5 2.Rh2 d5, I would leave it Blank.
While what post 2 says is the rule, it is implied that game data is allowed, which are all the fields at the top of the score sheet, which includes:
Event
Date
Round
Board
Section
Opening
White Name
White Rating
Black Name
Black Rating
Time Control