Donation Form - Stripe API key needed

I have two forms that should each collect payment. One of them works perfectly. The other generates the following error:

I have the API key from Stripe, but have no idea where I need to put/set that! I have this second form pretty much set up exactly like the other one (except it's donation versus product selection) and I'm not sure what the problem is. Can I only have one form connected to Stripe or something? Or can you guide me as to where I need to put this API key?

Try disconnecting and reconnecting the integration, see if the problem persists.

If that doesn't work, try switching to a different payment gateway first, then switch back to Stripe, and redo the integration.

The API is automatically generated when you connect your account.

Let us know how it goes.

Thanks.