If the domain purchasing process fails for any reason the domain can be locked or shown as "taken" for up to 24 hours during this payment pending state (depending on payment method).
For domains locked from a crypto payment attempt, you can retry after 2 hours. For domains paid using your card (Stripe) or Paypal, you should be able to reattempt purchase within 24 hours.
Please try purchasing the domain again after the lockout period ends. If the problem persists past the given timeframes above, please email support@unstoppabledomains.com so we can investigate the issue for you.