Xcode 12.5 on macOS 12: Code signing "MyAppExtension.appex" failed.

Seeing the same thing here. I'll dupe your FB.

have the same issue 😦

Can you please share how to boot back to big sure?

Same here

Having the exact same issue!

Same here

Same for me (feedback filed in FB9165736)

Updating to Xcode 13 Beta solves the issue perfectly. If you are on macOS 12 Monterey Beta please update to Xcode 13 Beta.

Also reproduced and filed under FB9171462.

Confirmed as working if you update to Xcode 13 Beta 1.

Curiosity - you can upload from Xcode 13 to App Store connect, but for internal testing/testflight only. If you try to submit it to the App Store it will get rejected. Even if you build it with Xcode 12.5 and upload with 13 as it will sign it with Xcode 13 (I had this issue last year with Xcode 11 built, Xcode 12 Beta upload)

Hopefully Apple will resolve this soon :-)

I've made a small research and it appears that based on what's happened in the past you can't release production apps to App Store from either beta macOS or beta Xcode. Such apps will be rejected during the review. That makes a little bit of sense to me.

Still I solved the problem for myself with very little pain preserving the macOS Monterey Beta installation. Here is the solution (a bit verbose):

• Download macOS Big Sur from App Store (right from your macOS Monterey Beta). The download link could be found here: https://support.apple.com/en-us/HT201372. No need to create a bootable USB flash. Just click the link and the Big Sur installer will start-up automatically when the download is finished;
• While downloading go to the Disk Utility and create a new partition where you can put your Big Sur (consult Google if in doubt, keywords - dual boot macOS);
• While downloading copy your Keychains folder from your macOS Monterey Beta (Finder -> Go -> Hold Option/Alt to reveal hidden items and select Library and then locate the Keychains folder) to a USB stick. You would need it in macOS Big Sur. Copy your coding project folders or other important files to the same USB if needed;
• Once Big Sur download is done and the installer opens up - simply install it onto the newly created partition. Right from macOS Monterey Beta;
• Once the Big Sur installer asks for a restart - get ready to hold the Option/Alt key upon system startup to select the Big Sur partition as boot device. Alternatively you can just change it in System Preferences -> Startup disk in right from macOS Monterey Beta;
• Once the Big Sur installation is done - copy the stored Keychains folder you saved before from Monterey to the same location in Big Sur.

Thank you!

Seems to be fixed with macOS Monterey Beta 3