I can understand the urgency to get this resolved! Not having your checkout working can be really stressful.
What payment gateway are you using on your store? Have you checked your account under Settings > Payments to ensure that the gateway is setup properly? I would also try disconnecting it and reconnecting it as well to see if that resolves the issue.
It's also important to not place a live test order on your store yourself. This can create a flag with your payment gateway and cause issues with processing payments. If you need to test your checkout you can do so by following these instructions: Placing a test order.
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