Oops Error in domain registration

[MihanHosting]

Hi
i have problem with latest version of whmcs 7.6.1 
when i want register domain from domain registration section in whmcs i saw below error :

Oops!
Something went wrong and we couldn't process your request.

Please go back to the previous page and try again.

exception 'Whoops\Exception\ErrorException' with message 'Call to a member function pluck() on array' in /home/portalm/domains/portal.mihanhosting.ir/public_html/cart.php:0
Stack trace:
#0 /home/portalm/domains/portal.mihanhosting.ir/public_html/vendor/whmcs/whmcs-foundation/lib/Utility/Error/Run.php(0): WHMCS\Utility\Error\Run->handleError(1, 'Call to a membe...', '/home/portalm/d...', 0)
#1 [internal function]: WHMCS\Utility\Error\Run->handleShutdown()
#2 {main}

I try  upload a fresh copy of the /vendor/ directory for the WHMCS version I have installed to my WHMCS installation directory but the  issue not resolved 

what is the problem ?
and how can i solve this problem ?
thanks