Heroku: Cannot run more than 1 Free size dynos

Question

I was trying to run

heroku run rake db:migrate

And was getting the error

Cannot run more than 1 Free size dynos.

Answer 1

The answer is to look for any open heroku sessions (you can use 'heroku ps' as john points out below), in my case I already had a started a heroku console session 30mins earlier and just forgot about it. So if you see the "Cannot run more than 1 Free size dynos" error just close any existing console or other heroku sessions you have open.

Hopefully this saves someone the ten minutes it took me to come to my senses.