For anyone else who comes across this like I did and is looking for an answer:
- Log on to another non-DC in the domain
- Log on as a domain admin
- Install/add the RSAT tools (the AD ones in particular)
- Launch the PowerShell, Import or install the AD module
- Run the Add-KDSRootKey from the new machine.
I don't know why you can't run this on the DC. However, any of the KDS powershell command give faulty information when run from the DC. Mine reported there was no key even, when there was. However running it from a non-DC on the domain will work. Maybe someone else can point out why the commands don't work on a DC. MS docs also don't say anything about running it from another machine.