I recommend you to contact Nop-Templates - they have developed the plugin itself. I also met a similar problem, but on 3.90, even thought the plugin wasn't useful for my case.
Nop-Templates staff are pretty active on nop forums...
Did you solve it?
Unfortunately, no, but I am keep working and I believe I will finish before Christmas. The API plugin by Nop-Templates is good for integrating Nop with your web store, but I need to integrate Nop with another similar loyalty platform, so it is different a much more complicated, but I am on the right direction. Thanks for asking and success with your work!
I wonder where's the version nopcommerce 3.9? I downloaded from github however; the version is for .net core. How and where can I download the version of this api for nopcommerce 3.9?