Please note, the question should be... "Hello, I'd like to know where the mistake is in this phrase."

If you say so, who am I to not agree?"

"If you say so who am I to not agree" <-- lacks a full stop. Otherwise seems technically correct. If you say so, who am I to disagree. <-- somewhat more natural. Might be some debate about the best punctuation to use after "so".

thank you so much

"If you say so, who am I to disagree?" The way you wrote it works too, but it definitely needs a comma and this way sounds better.