API - post /api/categories - don't work

Hello friends, im trying to create a new category with Postman, like it is written in the Documentation

But instead of creating a new category, I get a list of all categories as result. Do you know what the problem could be ? My middleware python scripts require this function to work properly.
How could I fix the API ?

API Key, User and password are all correct. And other functions are working fine.

Im struggling with this issue for a long time now…