The Forecast cap includes tax and unresolved automatic payments as well as confirmed Forecast purchases. An uncertain payment reserves budget until resolved, including across a UTC month boundary. The cap is different from your credit balance and number-spending limits.
Check the preview's remaining budget and any pending payment. A remaining amount smaller than the minimum tax-inclusive refill cannot fund a purchase. Review the cap or buy credits manually if needed; a manual purchase is separate from the Forecast cap. Ask support about an unexplained reservation using the existing purchase reference.