Same here on iOS 14.4, macOS 11.1 and Xcode 12.4
When the UDID is copy pasted with all caps I get
Starting device <My UDID> [FAILED]
For a laugh I tried changing the letters to lower-case and get no stdout, also tried -l and -L after each and "Could not get list of devices"
I'm just starting out on Apple development so I might've missed something though...
Edit: I've tried with and without sudo
thekingofjibs
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I tried that one and I get
/Library/Apple/System/Library/LaunchDaemons/com.apple.rpmuxd.plist: service already loaded
Load failed: 37: Operation already in progress
:(