I tried to fellow all steps in the documentation that you provoided in : 

Paypal Marketplace

but still have a problem to connect the Marketplace Account & also the Vendor account , & I don’t know what is the problem !! 

there is something missing but I don’t know where & what !! But it seems that it is caused by the Plugin itself (WCFM PayPal Marketplace ) I think that is because Paypal marketplace was updated but the pugin was not updated to be adapted with PayPal Marketplace changes !! 

Could you please help me to solve this issue ? 

here are the tries to connect with the sandbox accounts both (Personal & Business Sandbox accounts did not work & it is impossible to connect vendor’s account via WCFM !!