I'm getting this error too — even though I'm on XCode beta 5 running on macOS 26.0 Beta 5. Made the icon using Icon Composer. Can't upload the archive to App Store connect for a TestFlight due to this issue. ["Invalid large app icon. The large app icon in the asset catalog in can’t be transparent or contain an alpha channel. For details, visit: https://developer.apple.com/design/human-interface-guidelines/app-icons.]

@DTS Engineer Thank you so much for your reply. The issue persisted on Beta 5 so used the old-style icon export into Asset / AppIcon in place of the nice Icon Composer-made icon. But, I will test on Beta 6. If it still persists, will file a bug report. Looking forward to this being resolved in an upcoming build. Have enjoyed using Icon Composer - it makes creating wholesome icons a breeze. —Siddharth