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Make sure you setup your Test public key on the page and use approved Test Cards from Stripe.
https://stripe.com/docs/testing
This should work appropriately.
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I know this question was posted a while back, but I faced the same issue, so posting the answer here in case it is of help:
You need to check if the coupons were created in Live or Test Mode. When working on a local server, only Test Mode coupons and vouchers are applied.
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