If you're using Stripe's test mode (enabled through Stripe), then you'll need to follow their guide on simulating transactions and use their test card numbers. However, if you're using test mode on Shopify, then there are two different test modes that you can use for simulating transactions. Both of these methods also have their own test card numbers that you'll need to use.
If you're ready to start selling and processing orders on Stripe, then you'll need to disable test mode in order to process real orders.
Just to confirm, did you enable test mode on Stripe or Shopify? Also, did you get a chance to try any of the test card numbers or are you trying to process an order using a valid credit card?
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