You should only get that if you are using versions of Formidable Pro and Formidable Free that are not matching numbers.
I install nulled version of v5.5 Free now and keep v5.4.5 Pro I would get similar, get this - 'Warning! You are using an unofficial copy of Formidable Forms that may contain malware!' However, I go back to v5.4.5 Free or update to v5.5 Pro and those messages will not be shown.
As for Stripe connection, it does look like there is an issue with both the test and live links to stripe, have you tried using the additional code that they state on their own page here, as the nulling might make it inaccessible through normal method of simply click and link.
If you are not the account owner and only have developer access, you can use this code snippet to use the API keys for integration.
Integrate your WordPress forms with Stripe to process your one-time and recurring payments. Stripe forms enhance the user experience by allowing users to stay on your site while securely making a payment. As of Formidable Stripe version 3.0, the add-on has been updated to accept payments with...
formidableforms.com