Assigning money but not sure where it is coming from?

I've been using YNAB for 6 or 8 months now and I think I have a good handle on things. But I have something I don't understand.

I created a new category and gave it a target of $180 for the end of this month (not recurring). I have $0 in ready to assign. If I type $180 in the assign box for this new category it goes green but does not ask me where to cover this from. Any thoughts here?