Error on Test Server Reachability

[drumstick77]

Hi all, I'm new to nginx and have just installed it on docker successfully.
However, when I'm trying to create an SSL cert for a domain that I have on duckdns, I'm getting this error.

"There is a server found at this domain but it returned an unexpected status code 400. Is it the NPM server? Please make sure your domain points to the IP where your NPM instance is running."

Can anyone please advise what have I done wrong? Thanks