"We don't need to eat."
Is it an error to use the pronoun explicitly, like "Sisi hatuhitaji kula"?
I think it can be used sometimes. =)
Swahili is a pro-drop language. As the verb usually already includes prefixes to indicate the subject and object, personal pronouns aren't strictly needed, and are mostly used for emphasis. The exceptions to this include cases when the copula ni (or its negative counterpart si) is used, as well as with the habitual form of the verb, which lacks subject prefixes.
From Wikipedia: https://en.wikipedia.org/wiki/Swahili_grammar