Is "submittable" a valid word to describe something that is eligible to submit?
Google Ngrams says the word is in existence since 1840s with a meaning similar to that which you are asking.
I would say it means "something which can be submitted"
However, a more commonly used word for this is submissible
Some examples of submittable:
therefore not more than any other submittable to an arbitrary power and extrajudicial proceeding
the discharging of Greenock's heirs to purchase New Wark being neither submitted nor submittable in itself, laying them under a perpetual servitude
It Is contended that, because this net contains an emergency clause, it was not therefore submittable to a vote of the people under the referendum