"She was not an actor."
Translation:Níorbh aisteoir í.
It could be Ní raibh sí ina haisteoir (though I don't know if that's included in Duolingo's list of acceptable answers).
ina haisteoir because words that start with a vowel take a h-prefix after the possessive adjective a when it means "her". "He wasn't an actor" would be Ní raibh sé ina aisteoir.
Because aisteoir starts with a vowel.
Note that this is the negative past tense of the copula, normally níor, but it becomes níorbh before a vowel. The negative past tense particle níor, used with regular verbs (and most irregular verbs) does not become níorbh before a vowel - níor ith mé, níor ól mé.