While "pratt" is a valid word in the English language, it doesn't hold the same slang meaning as "prat." "Prat" is a British slang term for a foolish or stupid person, and it is always spelled with one "t."
It's understandable that you might have encountered "pratt" as a misspelling, or perhaps it's a regional variation of the word that you're unfamiliar with. However, the standard spelling for the slang term is "prat."