You must have "Full Access" or "Limited Access - Financials" permissions to add payment terms.
"Payment Terms" allow you to define the organizational policies about how to accept payment. The payment terms created can be used anytime that particular policy is offered. When adding a new set of payment terms, define the number of installments, what percentage of the total is due per installment, and the window of time between installment due dates.
For example, if it's common to offer a policy allowing six payments due monthly on the 1st, only one set of payment terms with six equal installments and an installment window set to the "1st of each month" needs to be created.
Then, anytime an invoice is sent or registration is set up, those payment terms can be selected. For each resulting sale, the payment terms will generate an individual payment plan that honors the policy.
In addition to that payment plan, customers will always have the option to pay their balance in full at checkout. Use Due in Full to force the pay in full to be the only option.
Once a set of payment terms have been applied to a sale, the installment settings for those terms cannot be changed. However, the name and description can be edited at any time. Due dates can also be edited, and payment amounts can be redistributed on the resulting individual payment plans.