Try to re-enter your keys again:
1. Delete current keys entered in on plugin settings page. Make sure all keys field are empty, then click “Save Changes”.
2. Copy keys from your Stripe Dashboard. Make sure you put corresponding keys to corresponding fields. E.g. “Publishable Key” from Stripe Dashboard should be in “Live Stripe Publishable Key” field on Stripe Payments Settings page. Click “Save Changes” when done.