"That is not possible!"
Translation:Tio ne eblas!
I would think that should be allowed, but it ultimately depends on context.
"Tiu" by itself (without the noun it qualifies) usually refers to a person or living thing. In such a case, use of "tiu" doesn't make much sense here.
But "tiu" by itself can sometimes mean "that particular thing" in which case I would think "tiu" is justified.
In the end, it depends on context. Since it is technically a valid translation (though not the most common), I would allow it.