Welcome to the community.
According to their docs:
You may opt-in or mark a Contact as Marketable by including the following field in the request JSON with an opt-in reason. (This field is also shown in the complete request body sample.) The reason you provide here will help with compliance. Example reasons: “Customer opted-in through webform”, "Contact gave explicit permission.
If you set the field
Opt-In Reason it should fix the issue. I just tried myself and realized that there is a bug in the node. The parameter
Opt-In Reason is available under
Additional Fields. Sadly, when the node is executed the parameter is not included in the HTTP Request make to Keep’s API.
I just fixed it, and it should be available in the next release (We release weekly).