Opening from Training:

This opening:

es.lichess.org/training/opening/15437

One possibility suggested by the machine is e4, however black has three pieces to eat the pawn, while we have only two to defend it.
Maybe I miss something (I am beginner)

Before anyone else replies with the answer I suggest setting the position up on a board and looking at some ideas. You've stumbled upon a very instructive position.

mm maybe if black eats the pawn with his pawn, white can move knight a4 (threatening b7 in case black follow eating) ?

By a4 do you mean h4?

Also post the full line, not just the general stuff.