Unfortunately, our API currently does not support that TLD, as shown on this list: https://www.dynadot.com/community/help/question/API-supported
Please note that the list is constantly changing and being added to, and we will be sure to add your suggestion to the relevant team for consideration.
To get instant answers, please use our chat (under "Support" in the top right corner of our website).
King regards,
Team Dynadot
Adding .NL TLD to API