While building a package in AEM using crx package manager throwing an error "package build Undefined". But when I check under etc/packages/mypackages , package was created. Even there is no error or exception in error logs. How can I resolve this issue.
It can be browser issue just check the errors coming in the browser console and you can update to the latest version ,May be it can help. In the case if it is not helping disable some plugins as they can also affect CRX.
The suggestions below can be related to other types of errors:
It usually fails when some of the nodes cannot be added to the package.
It might also be the case of the permissions to nodes however if you work on the clear instance then I assume you are using admin creds. You can also check if any of the nodes is not blocked however you should get feedback on this in logs.
It can be also the size of the data might be very big, or the nodes can be somehow corrupted, but again it should not the case for fresh instance.