TM021 (Japanese: わざマシン021Move Machine 021), known as TM21 prior to Generation IX, is a TM introduced in Generation I. It teaches a compatible Pokémon a move. The move it teaches depends on the game. For compatibility, refer to the move's page.
TMs in the Mystery Dungeon series are named only by the move they teach. For example, Attract is the item that teaches the move Attract. Most TMs, however, correspond to a TM in the core series' contemporaneous games.
In Rescue Team DX, TMs can only be used once. Whilst the available TMs are the same as those in Pokémon Sun and Moon, the TM list order is the same as for Super Mystery Dungeon (that is, in the TM order for Pokémon Omega Ruby and Alpha Sapphire), with newly added TMs added to the end of the list.
In Elekid Incorporated, Silver mentioned that he had used TM21 to teach his TotodileFrustration in order to capitalize on the fact that he had only just stolen the Pokémon and it hadn't come to like him yet.
Trivia
Along with TM28 and TM43, this is one of the only TMs unobtainable in both Colosseum and XD outside of trades.