The problem is not about the AccessControlPolicyParameters parameter but about the Name parameter.
When setting things, you need to use TargetName or TargetIdentifier. The Name is holding the new name if you rename the relying party trust. So in your case, the proper syntax would be:
Set-AdfsRelyingPartyTrust -TargetName "RPon2019" -AccessControlPolicyName "Permit everyone and require MFA for specific group" -AccessControlPolicyParameters "MFA Users"
And you can use either DOMAIN\GROUP or the GROUP syntax if the group is in the same domain as the ADFS farm.