1. 'I have seen an advertisement of your apartment.' It is true or false?

This is a question about the fact.

Answer: I've no idea whether it's true or false. You could be telling the truth or you could be lying.

2. 'I have seen an advertisement of your apartment.' It is right or wrong? Is it correct or incorrect?

This is a question about the words and the grammar

Answer: It's wrong. It should be 'I have seen an advertisement for your apartment.'

If you want to ask the owner that's correct.

Is it correct?