Fixing bug on WordPress Plugin

I have a plugin to send notification order when someone placed an order from woocommerce or multivendor site. All works except the function to send notification to multivendor. There is an error like this: Fatal error: Uncaught Error: Call to a member function get_vendor_data_list_from_order() on string.

I'm not expert on PHP so i can't solve the problem. If any of you capable to solve this bug, please do a favor and bid this project. Thanks.