I go "negative bank" all the time, and I think the max negative you can go is only 20 hours, not 30, which is why you are getting the insufficient bank message. If you are -18 now, try two hours and see what happens.

I just put 'fill up' and it awards me as much as possible to get to the ALV, more than 5 hours if I'm that short, but I don't think I've ever seen more than -20 in my bank, so that's why I think that's the limit.