Iām not quite sure what you are doing here? And I have no idea what NPM would have to do with a DNS issue?
Subdomains of dedyn.io subdomains are no problem.
Where did you try to add the CNAME? At the domain apex (example.dedyn.io) or for e.g. test1.example.dedyn.io?
CNAMEs at the domain apex will not work because the DNS specification does not allow that.
The error you are getting implies that other RRsets exist for the name you are trying to set the CNAME at. Again that is not allowed by the DNS specification. You can either have a CNAME or you can have other RRsets.
What you can do is set a CNAME from your subdomain to the domain apex: test1 CNAME example.dedyn.io. ā provided there are no other RRsets for the subdomain test1. I have no idea whether that is what you want though.