Not using that syntax. but if you don't pipe the analysis server object through it works
instead of
$AServiceServer | Set-AzAnalysisServicesServer `
-FirewallConfig $conf `
-Sku $AServiceServer.Sku.Name.TrimEnd() `
-DisableBackup
use
Set-AzAnalysisServicesServer `
-Name $resourceName
-FirewallConfig $conf `
-Sku $AServiceServer.Sku.Name.TrimEnd() `
-DisableBackup