End turn step

No — the "end turn step" is not a window you can act in: when the turn player says they're ending their turn, nothing opens a priority window, so you can't play a reaction or activate anything at end of turn (FAQ 6113, 8978).

The Ending Phase is the Ending Step (at end of turn triggers and effects happen) followed by the Expiration Step, which heals all units, expires all "this turn" effects simultaneously, and empties both rune pools (Rule 317.2). Those steps resolve as Tasks with no discretionary actions granted to either player.

The only way to get to act there is if something is actually put on the Chain — a triggered ability firing "at the end of turn," or a legend/card that triggers in that phase — which gives you a Closed State to respond in with Reaction-timed cards. And note you can't squeeze in a Main-Phase-style action right before the turn ends either; off-turn priority only comes from a chain or a showdown.