It’s telling you that the customer you’re trying to charge doesn’t exist in your Stripe account (presumably because you cleared out all your data in Stripe)…
I assume you’re using the Bubble Stripe plugin… I don’t know much about it as I rarely us it myself, but presumably the call to Stripe is including the customer ID (as it’s saved in your Bubble app via the plugin), but the customer ID no longer exists in your Stripe account… hence the error…
So, all you’ll need to do is clear the Customer ID from the user in your Bubble app… but, I’ve no idea how you do that with the plugin, or even if it’s possible.
If it’s just test data, then you can just delete the User from your app database and create a new user.
UPDATE:
by the looks of This Thread (which has some fairly recent posts), it does indeed appear as if you can’t remove the stored Stripe Custome ID using the plugin, so you’ll have no choice but to delete the User… or, if it’s very important live data, you could try contact Bubble directly.