ERROR: language "plpgsql" does not exist

I just installed the PostgreSQL7.3.1 on a linux server and trying to create
a function .

I got the
ERROR:  language "plpgsql" does not exist

Why?  How can I solve this ?

-- script
CREATE FUNCTION sales_tax(INTEGER) RETURNS REAL AS
 '
 DECLARE
        subtotal ALIAS FOR $1;
 BEGIN
        return subtotal * 0.06;
 END;
 ' Language 'plpgsql';


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Best Regards

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Michael Shi             Database Developer