[50$ For solution] Having a problem when trying to connect Billgang API

I am currently working on a custom plugin to fetch products from our Billgang store to Wordpress but I am getting an error when trying to connect.
support of billgang is not active. if you have an idea how resolve this problem I can pay a nice fee.

Error:
invalid JSON response

Screenshot added

Hello,

Happy to take a look at this for you, if you can raise your budget a bit.

Have a lot of experience working with BillGang