Adding VDAs to MCS machine catalog fails
Exception noted in logs: The argument cannot be bound to the parameter "MachineName" because it is an empty array.

CVAD service SDK was installed
https://www.citrix.com/downloads/citrix-cloud/product-software/xenapp-and-xendesktop-service.html

The following commands were run
asnp citrix*
Get-AcctIdentityPool -IdentityPoolName <Identity_pool_name>
Get-AcctADAccount -IdentityPoolName <Identity_pool_name> | ft ADAccountSid, AdAccountName, State

Several machines were listed as available and without ADAccountName
Active directory was checked for accoutns with name pattern from the IdentityPool, ensure only accounts in use were listed.
 

Additional powershell command was run individually to remove each "available"  ad account
Remove-AcctAdAccount -ADAccountSid <S-1-5-21-….> -IdentityPoolName <Identity_pool_name>

Get-AcctIdentityPool -IdentityPoolName <Identity_pool_name>
Now showed 0 available accounts.

Citrix AD Identity Service was restarted on DDCs (to prompt a quicker refresh of any cached data, this step may not have been needed)
Adding VDA to the MCS machine catalog was then succesful.