Personal account users and Heroku Team admins and members are asked to authorize additional charges for Review Apps and Heroku CI used in their pipelines.
To provide authorization, go to the Pipelines page for any of your personal or team's apps. Click the button in the banner to authorize additional charges. You must provide separate authorization for your all personal pipelines and the pipelines for each team you are an admin of.
See our blog post and the Removal of Heroku Free Product Plans FAQ for more info related to the ending of free plans.