Build for iOS: Getting Error - Failed to sign

How does Build > iOS Ad Hoc (.ipa) work?

I'm using MIT App Inventor as a standard registered User directly on my iPhone within the Safari app. So far everything with the project worked fine, also when testing it on the same device with the companion app which I installed from the AppStore. So I thought the project might be ready for the next step, building and installing it on the device as an Ad Hoc IPA.

Therefore I correctly set up everything on the Apple developer portal, using my already existing distribution certificate (since I already use it for other app projects and also on my iPhone in Safari the menu item "Download Certificate Request" is missing. Why is it not there? EDIT: Meanwhile I found it - it is only present in the classic user interface). I prepared the identifier and the Ad Hoc provisioning profile, which I uploaded to the media section of the project. Then I clicked on Build > iOS Ad Hoc (.ipa) and it immediately started to build the project. Everything seems fine until after some time it stops building with following Error:

[SignPackage] ERROR: Failed to sign framework /path

I also tried to create a new project just with a text box in order to test the building again, but it always fails with the same error. What I don't understand is how signing the IPA internally works, since normally I would also have to provide the distribution certificate itself (eg. together with the private key as P12). But App Inventor never asked me to upload this somewhere. Is it required to use App inventor on a Mac computer to successfully sign the IPA? Is Xcode required to sign it? Do I have to use a distribution certificate that was created via the "Download Certificate Request" CSR or can I use my already existing one? Or is the beta still only available to a closed group of users? Or what is the problem and how can I solve it? Can I just build and export the IPA and sign it somewhere else?

This is a key step so we should unpack what's going on here. In particular, the App Inventor workflow assumes that you won't already have done iOS development before and will need to set up a distribution cert. While it's technically possible to start from an existing certificate I have not written the documentation for this.

As for the missing menu item, you must be using the iosbuildservertest.appinventor.mit.edu server and it must be in the Classic UI (I haven't yet updated the menus for the new UI). You can temporarily switch back to the classic UI to generate the certificate.

Have you already reviewed the build documentation?