Redirect user to custom thankyou page after order complete through custom gatewa

Hi,

I am using WPLMS theme in my wordpress in order to teach some courses. However, I needed some custom Payments Gateways for my country, so I installed the plugin Custom Payment Gateways, so after making gateways I thought to test it, I gone to course, clicked on Take this course, it successfully took me to the Add to Cart page, I clicked on Add to Cart, then it successfully took me to the Checkout page, I I then filled billing info and select the Payment Gateway which I made custom, and clicked on order now, so it redirected me to the Add to Cart Page. I want it to redirect to the custom ThankYou page I designed.

And I have two more questions:
1. How can I simply allow any single user to get the course without paying the amount?
2. When I gone to WP Admin > Plugins > Installed Plugins > Custom Payment Gateways > I clicked on Settings and it gave me the error “You do not have sufficient permissions to access this page”, why this error came? And how to solve it?

Pleasee anyone help me in these three questions I posted above. I would be very thankful, I have less time, because the time I gave to students is running up, and I am unable to manage these things.