Hi sns,
As stated in the error, 'resourceType' cannot be null. Here is an example with resourceType:
New-AzConfigurationAssignment -ResourceGroupName smdtest$location -ResourceParentType hostGroups -ResourceParentName smddhg$location -ResourceType hosts -ResourceName smddh$location -ProviderName Microsoft.Compute -ConfigurationAssignmentName $maintenanceConfigurationName -MaintenanceConfigurationId $maintenanceConfigurationCreated.Id -Location $location
If this is helpful please accept answer.