A piece is missing.
'Manca' — MAN-ka. Short and sharp.
Use for items sold in multiple parts — furniture, appliances, games, model kits. State it immediately and check whether the missing part can be sent separately before deciding to return the whole item.
'Mancare' means to be missing or lacking. 'Manca' is third-person singular — 'a piece is missing' (literally 'a piece is lacking'). This is an impersonal construction. For multiple missing pieces: 'mancano dei pezzi'.
Non c'è il manuale.
The manual is missing.
Specific type of missing component.
Mancano delle viti.
Some screws are missing.
Plural — 'mancano' not 'manca'.
La scatola non era completa.
The box was not complete.
Focuses on the package.
IKEA Italy and many large retailers will post missing parts directly to your home without requiring a full return. Always ask 'potete spedirmi il pezzo mancante?' before making a trip back to the shop.