When you need to set up one of your brand new macOS Big Sur computers as a test device (as described in "Register Test Devices" in a previous post), be aware that Apple has changed the required value for current devices. When you visit System Report -> Hardware, there are now two entries in the Hardware Overview: Hardware UDID and Provisioning UDID. You must use the Provisioning UDID when you register your device on your developer device list. For Macs that have been upgraded to Big Sur, these two IDs are probably the same. But on a fresh new Big Sur device, they most likely will not be the same.
If you try to use the incorrect Hardware ID, you will probably get a cryptic error message saying something like "You do not have permission to open the application". (See this StackOverflow post.) It's non-obvious, but if you look at the console for the system log, you might see a message saying "Provisioning Profile Validation: profile 'XXXXX' is not provisioned for this device (<private>)". That's a good indication that you've used the wrong UDID.