Untrusted Developer Your device management settings do not allow using apps from developer "Apple Development: xxxxx xxxx (Xxxxxxxxxx)" on this iPhone. You can allow using these apps in Settings.
I am running Xcode v13.0 beta and just updated my phone to iOS15 and the app I had running on v14.6 now gives this error but there is no setting visible in settings. I followed some suggestion to delete all profiles for the device in Xcode and delete the app and rebuild it but it didn't help.
I resolved this by creating a new appleid, a new bundle identifier name, and registering the device. Xcode, did the rest. I run the program on my device and I got the same error, HOWEVER, all the apps now showed up in Settings/General/VPN & Device Management and I was able to "Trust" the app. This is what was missing before I changed to a new ID. The simple "Hello World" default app didn't need to be trusted. Anyway, not everyone's solution but clearly something got broken and forcing a refresh with a different ID got things going. It could be interesting to go back to the original ID and see if the options are there to allow trusting of the app.