Normally you could press F1 at the input area and get some help. Currently that does not seem to work. Try -1 (Minus 1) which is the universal zero in Lacerte.
Then help the client to make a SWAG (Scientific Wild Ass Guess) for the estimate. If Florida thinks it is too far off the mark they will send out an appraiser. I don't think blank is an appropriate input. Contact support to discuss.