The problem is solved. It was initially set up with beta4, but the archive wasn't recreated after using beta6, so it couldn't be uploaded.

I'm using Xcode 27 beta 6 and I'm unable to upload to the App Store and TestFlight. Unsupported SDK or Xcode version. Your app was built with an SDK or version of Xcode that isn’t supported. Although you can use beta versions of SDKs and Xcode to build and upload apps to App Store Connect, you need to use the latest Release Candidates (RC) for SDKs and Xcode to submit the app. For details on currently supported SDKs and versions of Xcode, visit: https://developer.apple.com/news/releases.