"Repository not found or permission denied" when creating AWS Codebuild Repository

I had this issue because I was using a low-privilege account to access GitHub. CodeBuild needs admin access to the repository in order to create webhooks, so the solution for me was to grant the user admin access to the repo in question (via GitHub, in the repo's Settings page), create the CodeBuild project, then set the user back to read-only.

Try:

CodeBuild > {project} > Sources > Disconnect from GitHub, then Reconnect with a GitHub account where your code repository is.

CodeBuild can only hold one GitHub authentication per account per region.