A letter/email of apology to friend / acquaintance for not being able to refer client / customer / contact in business.
I am sorry to inform you that I am unable to refer you to any potential [contacts / customers] for your business as [I have not been in this industry for many years] / [I do not know anyone in this business] / [I do not know the people in this business well enough to make an introduction] / [I am not aware of anyone interested in your service / products].
accept our apology,
unable to help,
apology mail to client,