Solution maybe here at this thread
https://forum.gsmhosting.com/vbb/f11...mdisk-3093815/
Just go fast to the justatech site or here:
https://idevicecentral.com/ios-signing-status/
and find the link for the IOS 15.XX beta firmware and write it with 3utools, as long as the signing process for those firmwares is still working by Apple...