Can you check the build logs on sprintr.mendix.com → your application → Environments → Deployment packages. I assume there will be a failed build listed there.
Edit: there appears to be an issue: https://status.mendix.com/
Investigating - We are currently experiences issues with a building free/sandbox applications that affect a portion of our customers. Running apps are not affected, and paid apps are also not affected. We are investigating the root cause. Apologies for the inconvenience this causes.