If you see an error upon charging the guest's card in stripe that says "CARDFAILSTRIPE Card not accepted by stripe"
You can check the exact reason when you log into your Stripe dashboard and click on the left menu Developers > Logs, here you can see the bookings and details that failed on Stripe and the exact error.
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If in the bottom part of the error message shows like this :
Request POST body
"error": {
"message": "Sending credit card numbers directly to the Stripe API is generally unsafe. To continue processing use Stripe.js, the Stripe mobile bindings, or Stripe Elements. For more information, see https://dashboard.stripe.com/account/integration/settings.",
"type": "invalid_request_error"
​
What you can do is login to your Stripe account and go to Stripe>Business Integration>Process payments unsafely

Or you can use "ChargeAutomation", BookingAutomation's payment processing partner.