Unknown table or alias 'A1'

Unknown table or alias 'A1'

I would like to create a subquery but i am getting the following error: Unknown table or alias 'A1' used in select query.

This is the sql statement: 
SELECT A1.active_paying_customers, A1.active_trial_customers, A1.new_paying_signup, date(A1.date_active_customers), 
(SELECT SUM(A2.new_paying_signup) FROM "Accounts Active" A2 WHERE date(A2.date_active_customers) > date(A1.date_active_customers)) as Sum_seven_days
FROM "Accounts Active" A1