Hey, @anthonyflibustiers!
Thanks for clarifying!
This issue is most often caused by either 1) API keys being mis-copied or entered in the wrong fields, or 2) a conflict with code in the active theme or another active plugin on the site.
As a first step, I’d recommend re-copying both the public and secret keys from your Stripe dashboard into the Stripe gateway settings on your site. Make sure you don’t miss any characters when copying/pasting, and ensure they are copied into the correct fields —?Public/Secret — and also the correct mode as Live keys and Sandbox keys will not work in the opposite mode.
If that does not solve the issue, I’d like to understand your site properly. Please share with us the necessary information below for us to investigate the issue further:
System Status Report which you can find via WooCommerce > Status > Get system report > Copy for support.
Fatal error logs (if any) under WooCommerce > Status > Logs. Then you can use the selector to search for Fatal Errors.
You could copy and paste your reply here or paste it via https://gist.github.com/ and send the link here.
Please let us know how it goes.
Have a wonderful day!