Issue:
When trying to link a customer account to an existing Master Account, the Master account is not showing in the drop-down list to be selected.

Resolution:
The problem could be from a mismatch between the Customer and the Master Account record for the Master Account.
When the application looks for valid Master Accounts, it checks to see if the Customer is active, and the Master Account record is active.
If the Master account is set as inactive, the account will not show in the list.
The script below will show records where the Customer Account for the Master is active, but the Master Account record is inactive.
SELECT ma.Inactive,cst.Customer_Status_Id,ma.Master_Account_Id, ma.Master_Account_Code, ma.Description, cst.Customer_Number, ma.Master_Account_Billing, ma.Last_Cycle_Date, ma.Inactive, ma.Account_Index FROM AR_Master_Account ma INNER JOIN AR_Customer cst ON ma.Customer_Id = cst.Customer_Id AND cst.Customer_Status_Id <> 3 WHERE ma.Inactive = 'Y' ORDER BY ma.Master_Account_Code
The record can be updated based on the Master_Account_Id
update AR_Master_Account set inactive ='N' where Master_Account_Id = 53469