Question

What are the moves that sketch can't copy?

Comments

But just keep in mind that you must fight an enemy that knows that move, in order for it to be copied.

Answer 1

In Generation II, it could not copy Selfdestruct, Explosion, Transform, Struggle, Metronome, Mimic, Mirror Move, Sleep Talk, or Sketch.

In Generation III, it could not copy Struggle, Chatter, and Shadow moves. Selfdestruct and Explosion could only be copied if a Pokemon with Damp was in play.

In Generation VII, it could not copy Z-Moves, Struggle, or Chatter.

In Generation VIII (excluding BDSP), it could not copy anything, as it cannot be selected in battle.

In Generation IX, it could not copy Dark Void, Hyperspace Fury, Revival Blessing, Tera Starstorm, or the Starmobiles' Torque moves.

Sketch (Bulbapedia)

Answer 2

Struggle wont work, unless your PP is all gone. All other moves will work.
If Mentronome or Assist is used, the move that the move uses is Sketched, for example, if Togepi used Mentronome that appeared with Signal Beam, Signal Beam would be Sketched, not Menronome or Assist.

Answer 3

Chatter and Struggle.
Some moves must fail in order to be Sketched