'George said that he made his house clean before the party.'
is grammatically correct. However, this sentence is in the active voice instead of the passive voice; perhaps this is the error your teacher was pointing to.
Here is an example of a version of your sentence, but in the passive voice:
'The house was cleaned by George.'
We know that this sentence is in the passive voice because it has a form of the verb 'to be' (in this case, 'was') followed by something that is technically called the past participle (in this case, 'cleaned').
There are other ways we could implement passive voice:
'George said the house was cleaned by him.'
Here, the main phrase of the sentence is in active voice ('George said'), but the additional information is in passive voice ('the house was cleaned').
'It was said the house was cleaned by George.'
Here, both the main phrase and the additional information are in passive voice. However, it is worth noting that phrases like this are rarely used in everyday conversation, as it is relatively abstract (the 'it' in this sentence represents people in general).