heroku ps
the result is some like this:

run.4859 (Free): up 2016/01/12 21:28:41 (~ 7m ago): rails c
So the numbers 4859 represent the session that is open and needs to be closed. To fix the error you need to run(Obviusly, replace the number 4859 by the number obtained):

heroku ps:stop run.4859