What happens if a backlog item doesn't have a rough estimate?
You can assign backlog items to releases even though they don't have rough estimates. This is particularly useful when you're doing the loose planning of a release and you haven't gotten around to adding rough estimates on all backlog items.
If the release is in progress, it's a good idea to add rough estimates as soon as possible, since the release burnup chart and the release date prediction will not take backlog items without rough estimates into account, so will be less accurate if some backlog items are missing rough estimates.
In the release burnup chart, the lines for planned work and done work will be dotted if there are one or more backlog items without rough estimates on a given day, since this means that the lines are less accurate because of the missing estimates.