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Hi,
i was also having same issue at open cart v2
i have taken fix ... there are changes in two files contact.php and contact.tpl
The Fix change Sale to customer in following line of code from contact.php
$this->load->model('customer/customer_group');
$data['customer_groups'] = $this->model_customer_customer_group->getCustomerGroups();

where as it was sale/customer_group in pre fix version of contact.php.

There is no Customer directory in admin/model
Other than contact.tpl and contact.php...is there fix in some other files and directories too?
Or i need to create directory customer and move existing customer_group there
kindly comment!

I fail to see where you notice these lines. I'm using v2.0.3.1 as I don't notice these lines anywhere in OC. As to answering your latest question, no, you do not need to create additional folders inside the sale folder nor inside any folders by default.