Can't assign ad to line in iAd Workbench

I've made a banner ad in iAd Producer, and uploaded it successfully to iAd Workbench. When I go to 'create an ad' and select 'from iAd Producer', the ad shows up and I can select it.

However, when I go to 'assign your ad to lines', the checkbox turns my cursor into a red 'no' sign and won't let me click.

Why is this? Why can't I assign my ad to the line? Why isn't there any error message to explain it?