Create a managed service account

Hi,

I am trying to create a managed service account on a domain controller in AD using powershell to use across the domain.

I am using the first example here https://blogs.technet.microsoft.com/activedirectoryua/2009/01/29/create-managed-service-accounts-with-powershell-do-not-create-managed-service-accounts-with-active-directory-users-and-computers/

I enter the code and then I am asked for the DNSHostName which I enter the FQDN of the DNS server/AD Server

I receive the error:

New-ADServiceAccount : Key does not exist
At line:1 char:1
+ New-ADServiceAccouint -name NameOfAccount -Path "CN=Managed Service Accounts,DC=IM .....
+ ....................................................................................................................................................................
     + CategoryInfo     : NotSpecified: (CN=AccountName...EndOfDomainName,DC=local) [New-ADServiceAccount], ADException
     + FullyQualifiedErroID : ActiveDirectoryServer:-2146893811,microsoft,activedirectory,management,commands,newadserviceaccount

What am I doing wrong?
Thanks
Gareth