Why is my email stuck in Audience calculation in progress" status?
There is currently a highly escalated bug (69516) that causes this error to happen. It appears to be an issue with the task timing out and losing it's connection to the Database, and then terminating. This happens rarely, but it may occur from time to time until the issue is resolved.
RDO is working with engineering to resolve this issue, but for the time being if you see this happen contact support and let them know to reference the case to bug 69516.