After I´ve done my tax return I received a return payment what is nearly 800CAD less then it said here on my profile it would be. How can i get an answer for the reason?

You need to contact CRA. Read your 2018 ;Notice of Assessment for possible reasons.

thanks for your reply! the problem is that i can`t access is it, like i can`t creat an account on the CRA page.
it always comes up that one of my details is incorrect but it actually isn`t!