your query does not include the specified expression as part of an aggregate function, how to...

How do I correct this code to fix the error I'm receiving of "Your query does not include the specified expression as part of an aggregate function"

SELECT CUSTOMER.ACCOUNT_NUM, CUSTOMER.CUSTOMER_NAME, CUSTOMER.BR_NUM, CUSTOMER.REG_NAME, ORDER.ORDER_TYPE, ORDER.COMP_DT, ORDER.ISSUE_DT, [ORDER]![ACCOUNT_NUM] & [ORDER]![ORDER_TYPE] AS [Key], INTO Final Output
FROM (ORDER INNER JOIN CUSTOMER ON ORDER.ACCOUNT_NUM = CUSTOMER.ACCOUNT_NUM)
WHERE (((CUSTOMER.INDQ)=0) AND ((CUSTOMER.PLAN_IND)=1))
GROUP BY CUSTOMER. ACCOUNT_NUM, CUSTOMER. CUSTOMER_NAME, CUSTOMER.BR_NUM, CUSTOMER.REG_NAME, ORDER.ORDER_TYPE, ORDER.COMP_DT, ORDER.ISSUE_DT,
HAVING (((ORDER.COMP_DT)=20210129));

Continue reading...