Corinna longum iter facit should also be ok?
Yes, this is also correct
Is there any reason my response "Corinna iter facit longum" was wrong?
"Longum" is an adjective that modifies "iter" and they should not be separated by the verb like that.
Why isn't "Iter longum facit Corinna" accepted?
Report it if you think it should be accepted. However, placing the subject after the verb can be confusing (especially one which is the same in the nominative and the accusative) and I wouldn't recommend it unless you are writing poetry.
"Iter longum Corinna facit." not accepted. Why?
Report it. It looks okay to me, although not a standard sentence construction.