It seems that you have not added the "listen" statement for IPv6 in the nginx.conf file of your domain. Without adding IPv6 to the nginx.conf file, you cannot make the Nginx server listen to an IPv6 address on the server.
Edit your nginx.conf file for the domain and add the highlighted statement inside the "server" statement.
listen IPv4:443 ssl http2;
listen [IPv6]:443 ssl http2;
Replace IPv6 with the IPv6 address of your server. DO NOT remove .
If you multiple IPv6 on your server, you need to add the following line, i.e., do not specify any IPv6 address:
listen [::]:443 ssl
Restart the Nginx server, and now the SSLlab test will not give any error.