If you’re unable to view the dropdown options under the external lookup field, ensure if you’ve enabled the following preferences in Zoho CRM. To do this:
Log in to the Zoho CRM web application.
Click the Gear icon in the top right corner.
Click Security Control under Users and Control.
Click a profile. Under Extension Permissions, enable ‘Zoho CRM API Access.’ Once this is done, perform an instant sync and check if the users can view the values while creating an expense in your organization.