早安, 爸爸 :-D
You said "fun" is uncountable. Correct. In "what a great fun", "a" would belong to "fun", but then, would it not mean that "fun" can be premodified by "a", in which case "fun" would be a countable noun?
"What a great fun" is incorrect.
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: