The answer is to look for any open heroku sessions (you can use 'heroku ps' as john points out above), 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.

Had the exact same issue and came to this page. After reading realized what was going on but want to add following.

just run

heroku kill DYNO --app your_app_name

After this close all open consoles.

Then run db migrate command, it will work.

Most Efective

in console run:

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

It is a very simple solution.

In my case, I ran heroku ps:restart to restart all dynos and the heroku run * command worked again.

Examples

If you just have one Git remote for Heroku, use this:

heroku ps:restart && heroku run *

If you have multiple Git remotes for Heroku, use this:

heroku ps:restart --remote your-remote-name && heroku run * --remote your-remote-name

OR

heroku ps:restart --app your-heroku-app-name && heroku run * --app your-heroku-app-name

Replace * with your command e.g. console for Rails console.

What I meant by your-heroku-app-name here is the sub-domain for your Heroku app. For example, if your app URL is https://cute-cat.herokuapp.com, that means your-heroku-app-name is cute-cat.

If you are not sure/forgot what's your Git remote name for Heroku, git remote -v can help you with that.

Example:

$ git remote -v
this-is-the-remote-name      https://git.heroku.com/xxx.git (fetch)
this-is-the-remote-name      https://git.heroku.com/xxx.git (push)
this-is-another-remote-name  https://git.heroku.com/yyy.git (fetch)
this-is-another-remote-name  https://git.heroku.com/yyy.git (push)

Just restart all dynos. heroku restart

For me there was one command running on the web run console, I just ended its session there and it was resolved.

Heads up: Heroku free tier is going away soon

"What happens if I take no action on my free apps or databases or do not upgrade to a paid plan?

free dynos will be scaled down to 0 and hobby-dev databases will be deleted starting November 28, 2022."

"Starting November 28, 2022, free Heroku Dynos, free Heroku Postgres, and free Heroku Data for Redis® plans will no longer be available. If you have apps using any of these resources, you must upgrade to paid plans by this date to ensure your apps continue to run and retain your data. See our blog and FAQ for more info."

REF:

https://blog.heroku.com/next-chapter

https://devcenter.heroku.com/articles/free-dyno-hours

https://help.heroku.com/RSBRUH58/removal-of-heroku-free-product-plans-faq