GitHub has a known issue

Incident Report for App Center

Resolved

The issues with App Center Build have been resolved and all operations are back to normal now. Thanks for bearing with us!
Posted Apr 21, 2020 - 17:18 UTC

Investigating

App Center is currently experiencing issues with GitHub repositories. Please also refer to https://status.github.com for updates. We apologize for the inconvenience while we work to resolve this issue.
Posted Apr 21, 2020 - 16:14 UTC
This incident affected: Build.