Strapi v4 Heroku deployment error R10 failed to bind to $PORT within 60 seconds of launch

I ended app creating a new app with the same files. It started.
Now i have setted up a staging app in order to have a backup of the stable. When i get the same error, i remove the problematic app, create a new one , and then copy the db from the stable one. I hope someone can point the real reason about the R10 issue so i would stop doing that.