Ancestral Katana

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2022-02-18
  Official Ruling
  If you choose to pay as Ancestral Katana's ability resolves, a second triggered ability triggers and players may respond to it. Then, as that second ability resolves, Ancestral Katana becomes attached to the creature that attacked alone.
• WWizards of the Coast2022-02-18
  Official Ruling
  A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Ancestral Katana's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

Rulings via Scryfall