First of all you should go with X-Cart 4.6.1 as it comes with much updated X-Payments connector module that comes with many improvements and fixes comparing to 4.6.0 and supports all perks of
new X-Payments 2.x.
Btw, what X-Payments version are you installing?
Internal error you see is caused by the fact that install.php script cannot connect to SMTP server and send install notifications by email. Please consult with your host admin to fix this or check what you entered as email/SMTP settings during previous steps of X-Payments installation.