@WELCH, Matthew As you are already aware that you use property aliases to access specific properties for a resource type. Aliases enable you to restrict what values or conditions are allowed for a property on a resource. Each alias maps to paths in different API versions for a given resource type. There are only certain set of aliases where you can use modify effect on. To find aliases that can be used with the modify effect, use the following command in Azure PowerShell 4.6.0 or higher:
Get-AzPolicyAlias | Select-Object -ExpandProperty 'Aliases' | Where-Object { $_.DefaultMetadata.Attributes -eq 'Modifiable' }
All the aliases which are trying to modify does not have modifiable property enabled.