Check memory usage on the box. I’ve often seen it complain like that when its struggled to build the instance. Or try clearing the git repo on the server.

Check on the stack trace just below the following lines, you’ll be able to see the error;

CHECKS file not found in container: running simple container check...
-----> Waiting for 35 seconds ...
remote: App container failed to start!!
=====> yourAppName container output:
/app/.meteor/heroku_build/app/programs/server/node_modules/fibers/future.js:245
throw(ex);

If the error is, Error: MONGO_URL must be set in environment. Then it’s evident that you haven’t created your database for the app.

If the error is, Error: ROOTURL must be set in environment. Then you’ll need to set-up your rooturl in the environment.
This resource might be of help, Dokku on DigitalOcean

Been running into this problem but figured out. Hope this helps. :)