Did you update dom0 beforehand?
You can install before using salt as per How-to: Install Qubes-Whonix
This would be a Qubes, not a Whonix issue. → What to post in this Qubes-Whonix forum and what not.
At very least the error message isn’t great. Please check Issues · QubesOS/qubes-issues · GitHub and report a bug if none open yet.
When reporting at Issues · QubesOS/qubes-issues · GitHub you’d probably be asked to provide it so I’d post it upfront.