Forecast needs an eligible saved card or PayPal payment method for variable refills. Pix and UPI use approved fixed-amount schedules or manual purchases instead. Seeing the Forecast option does not mean the selected payment method supports it.
Use Scheduled or Manual for that method, or review switching the automatic agreement to an eligible card through Billing. A manual card purchase does not change the agreement used for automatic charges. If Billing reports that the saved method is no longer available, update it and review the Forecast settings before saving again.