Mangopay problem / other market payments systems

-- HikaShop version -- : 3.1.1
-- HikaMarket version -- : 1.7.3
-- Joomla version -- : 3.7.4
-- PHP version -- : 7

Hi,

I have a sandbox account to test Mangopay.

Hikashop (market) MangoPay Payment Plugin (with my client ID and Password, sandbox yes) ...is active

HikaMarket - HikaShop Integration plugin ... is active

in Hikamarket: HikaMarket - MangoPay integration ...is active

But when activating the Hikashop (market) MangoPay Payment Plugin for payments there are these error messages:
ERROR:
1. Failed to get/create the legal user for your main vendor.
2. Please check that you right set your MangoPay credentials in the plugin (Joomla site)
3. Please also check that your account have a valid published address

First question:
How can I solve these 3 errors? I do not understand exactly what is missing.

Second question:
Mangopay support is quite slow.
Is there another payment service for markets which you could recommend? Anything similar?


thanks