The main error is in your Schedule backend WF on a list that YT_age&gender should be set to This text. In backend WF, uncheck LIST for the key YT_age&gender
Hovewer, I believe you should do that:
The backend WF should be set of the type of the API list (Probably ending by "‘s’ rows as this is the list you have with the name of api before)
After, the backend WF should have 3 fields: age range (text), gender (text) and finally viewer percentage (number)
In the Schedule on a list, you will also set the data source to just result of step 1’s rows. Each field will be
This text:first item, This text: item #2 and this text item #3 (or last item). So you will have each data already split before sending it to backend WF. (I’ve included the viewerpercentage, but you can remove this if not needed.)