I fixed this error by adding [Organization]\Project Collection Valid Users as 'Contributor' to the feed's permissions.

It's a bit confusing. Some would say that this is too broad, but it's not worth my time to troubleshoot it. Shame on Microsoft for making it so complicated.

If you have some time and you'd like to apply a more granular permission, I recommend that you review this link: How do I set the build authorization scope for my project?