This error means that the master branch on Heroku contains commits that are not in your local branch.
You can either pull the missing commits from Heroku and merge them into your local copy:
git pull heroku master
Or, if you don’t care about the missing commits you can force push to Heroku. This will overwrite the remote repo on Heroku with your local commits.
git push --force heroku master
Make sure you really don’t care about them as you will lose them from Heroku by doing this. Normally this doesn’t matter as Heroku is not normally the canonical repo, somewhere else such as GitHub is.