It didn't look like that to me. When there are candidates without vote, the submit button was greyed out with a hint stating "Missing preference: x", or "Duplicate preferences: y".
You don't have to add a vote for every candidate. Those errors are if you are missing a vote in-between, for example, 1,2,3,4,6,7. Missing 5 would bring that error up. It would be ridiculous if you had to add a preference next to every candidate.
Guess it can't be helped; voting is supposed to have closed by now. I do hope it gets fixed for the next election.
This has already been fixed in the latest version of the council site. Changes weren't pushed due to being too close to closing, running the risk of compromising already submitted votes (it was a large code change). The council site was never developed to be able to change preference orders, we added that functionality in a day before elections began to avoid timeout issues.
To circumvent this issue, one had to clear all votes and submit, then renumber them in the desired order. Or, clear all votes onwards and resubmit. For example, if you wished to switch 13 to 14, you'd need to clear 13 and upwards, submit and then reorder.
Numbering above 10 preferences, would be pointless (7 is actually more than enough). If you wish to switch your votes around, you can PM (before 3 PM UTC, 27/05/16) me and I will manually change it.