Why installing i get the error "Unique Constraint Violation Found" - what to do?

Delete the payment method configurations in the Magento Database  

This issue can happen in case the installation failed or you used the plugin already in another context. We recommend that you delete all payment method configurations in the database of the webshop and then simply enter your credentials again as described in the Magento Manual. 

This should finally trigger the creation of the payment methods again.