Sudden deployment failure from private GitHub repository on Hobby plan

You’ll be able to deploy your project if you make the repo public or move it to a personal GitHub account.

Deploying private GitHub organization repos has been restricted for as long as I can remember, but a loophole was allowing some to be deployed anyway. A recent update corrected it.

From Deploying private Git repositories using Hobby teams:

You cannot deploy to a Hobby team from a private repository in a GitHub organization, GitLab group, or Bitbucket workspace. Consider making the repository public or upgrading to Pro.