Why can't I find the Payment Term from the Payment Terms drop-down in the Credit Application Form?
If you are not able to find an active Payment Term from the drop-down list when you set the Payment Terms for a customer in the Credit Form, then you may follow the steps below

You can check your current Active Payment Term Settings (Settings -> Credit Forms -> Payment Term Settings -> Display Payment Term).
Here, if it does not show or include "Credit Form" when you check the options under "Display Payment Term", this means that this term will not be displayed in the drop-down as an option to select on the credit form.
If you would like to add this payment term as an option to select from the credit form, go to Settings -> Credit Forms -> Payment Term Settings -> Display Term Settings -> Click on the space inside the box and select Credit Form from the drop-down -> Hit Save Changes.
This will allow this payment term to be displayed from the drop-down on the credit form in the future. Please see screenshot below for your reference.