Payment by PayPlug

-- HikaShop version -- : 4.0
-- Joomla version -- : 3.8.13
-- PHP version -- : 7.1.22

Hello,

I configured the PayPlug payment plug-in in Hikashop.
Here is an error message that I have:
Message
The file /home/xxxxxxxx/sites/xxxxxxxxx/media/com_hikashop/payplug_parameters.json is missing. This file is generated when you save the settings of your PayPlug payment method. So please check that the permissions are OK in that folder and save the settings of the the PayPlug payment method again.

Can you tell me why even though configure it can not create payplug_parameters.json?
thanks in advance
Mike