User is trying to create a new activity item and wants to associate an income account with it. However, the user cannot find the account in its Income Account drop-down.
This can happen if the income account has an inactive status in the Chart of Accounts screen. By default, Core does not display accounts with inactive status in the drop-downs. You need to change the status of the account to Active in Chart of Accounts > Account Details so that it is visible in the drop-downs. Check the Core Help for more on editing account details.