This took me a while to find.

My Amplify build/deployments were failing with the error message "Unable to assume specified IAM Role". The issue was the AWS Amplify Github App lost access to my Amplify Project's GitHub Repository.

Fix: In your GitHub Org, go to Settings > GitHub Apps > AWS Amplify and choose Configure. Review the settings in the section Repository Access. In my case, I had to select the GitHub repository.