What happened in my case was I had to check build logs - and make sure "All Issues" was selected near the top. I had "Errors Only" displaying before. I saw the "Failed to resolve package dependencies." warning. The message told me I had a duplicated package in Package.resolved. I removed the dupe and boom, we're golden. This was super frustrating because Apple should figure out how to categorize its errors (it's an error if it can't build) that are obvious and easy for the compiler to recognize.