If you could show the exact wording of the message then no doubt somebody here will recognise it.
Check your Blocks in the Viewer - bottom left hand corner, click on "Show Warnings". You might have a puzzle piece missing, a non-existent component or variable, any little error like that will cause an issue.
On the other hand, everything could actually be fine in your app but just at the time you tried to build it, the MIT Server or your Internet connection had a problem (both of those have happened to me).