Vital CRM
New business opportunities include transaction, offer, and failure, etc. Should they be placed in the "Opportunity" or "Opportunity Stage" of the custom category?

It can be placed in the "Opportunity Stage".
How can I set the success/failure reasons for opportunity closure?

You can add examples in the opportunity stage (as shown in the figure)


When adding an opportunity, you can select your custom stage from the drop-down menu.