Couldn't this be translated "One thing's for certain?"
Yes it can! Not sure why they're not accepting it
Couldn't this also be translated as "a sure thing"?
That would turn a sentence into something else.
No, because è = is ; so this sentence is stative--"One thing IS sure". "A sure thing", on the other hand, is not a stative sentence-- it is a modified noun. (article + adjective + noun)
They are very similar in meaning, but grammatically different.
A girl is not ready