Hallo, I understood that the noun should be put before the adjective so why is Un Chat Petite is incorrect

The situation where adjectives are placed with respect to the noun is not clearcut in French. Some adjectives even change meaning depending on where they stand.

You can check here, for example: https://www.dummies.com/languages/french/how-to-place-of-french-adjectives-correctly/

Because short and common adjectives go before the noun.