I see the issue. You are trying to use an Active List (note the “dynamic”: true that is part of the response body) It is not possible to manually add contacts to an active (dynamic) list.
You'll need to create a static list if you want to manually add contacts to lists — in-app or via the List endpoints.
Source — “Please note that you cannot manually add contacts to dynamic lists. To determine whether a list is dynamic or static, when you get a list, you will see a flag called dynamic that equates to true or false.”
I see the issue. You are trying to use an Active List (note the “dynamic”: true that is part of the response body) It is not possible to manually add contacts to an active (dynamic) list.
You'll need to create a static list if you want to manually add contacts to lists — in-app or via the List endpoints.
Source — “Please note that you cannot manually add contacts to dynamic lists. To determine whether a list is dynamic or static, when you get a list, you will see a flag called dynamic that equates to true or false.”