I can't find the reason why : riceful and antful are ungrammatical. Can someone help me? Thanks
I think you've misunderstood what grammatical means. The words you've cited are simply not used by anyone I've ever heard or read.
If they had a meaning they could be put in a sentence.
Let's give one a meaning: antful means the amount of something that could fill an ant.
You've got thousands of antfuls of milk there.
I have used that word grammatically.
A word without context is not ungrammatical, it's just meaningless.