I'm having this issue as well. Looks like there are two issues possible:
- Your Files tab is full of .xls files and will not create any more when the readiness report runs.
- The Optimizer is actually down.
In this case it looks like we're all facing the latter.
Similar issue here: https://developer.salesforce.com/forums/?id=9060G000000MR4Y
I'm going to look into notifying someone on the Developer Force forum while we're waiting on Lynn H's Case with SalesForce.
Edit: Seems to be a known issue right now. Looks like people are waiting on the Optimizer team to bring that service back up.