In this sentence, it should be "frustrated". He left the company in a frustrated mood. You can tell because "left" (past tense version of the present tense verb "to leave") indicates this happened already so you would say frustrated because its the past tense and past participle of "frustrate".

'frustrate' is a verb in the infinitiv. You can't use it in this place in the sentence,you have to use the adjective 'frustrated'.

yup. frustrated indeed.

'Frustrate' should be replaced by frustrated because frustrate is a verb and cannot be used in this way. The adjective is frustrated.