Converting T-SQL to PostgreSQL

I am trying to re write the following stored proc as a postgresql
function......but am new to postgres and it is kind of getting really
confusing trying to learn and let alone understand the syntax:

CREATE PROCEDURE [dbo].[GetAppAvailability]
@AuditAvailable bit output,
@BillingAvailable bit output,
@ReturnValue int output
AS
SET NOCOUNT ON

set @AuditAvailable = (select app_Status from AppAvailability where
app_Functionality = 'Audit')
set @BillingAvailable = (select app_Status from AppAvailability where
app_Functionality = 'Billing')

Set @ReturnValue = @@Error

I have this in postgres but obviously it is not correct:

CREATE OR REPLACE FUNCTION GetAppAvailability(OUT auditAvailable character
varying, OUT billingAvailable character varying)
AS
$BODY$
set $1 = (select "app_Status" from "AppAvailability" where
"app_Functionality" = 'Audit');
set $2 = (select "app_Status" from "AppAvailability" where
"app_Functionality" = 'Billing');
$BODY$
LANGUAGE 'sql' VOLATILE
COST 100;

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