You have it in the query twice with the same name.

update co_customer_x_address

set cxa_on_hold_flag = '1'

from

(select distinct a1.cxa_mailing_label, a1.cxa_adr_key, M.cxa_adr_key

from co_customer_x_address a1

outer apply

(select a2.cxa_adr_key from co_customer_x_address a2

where a1.cxa_adr_key = a2.cxa_adr_key) M

Where a1.cxa_adr_key = '4490a7ff-cbfe-4386-8643-ec8cec854410') A