I think you are using nginx >= 1.25.1, and hence, you are getting the warning message.
The fix is very simple. Open the Nginx configuration file mentioned in the warning message and make the following change:
From
listen [IP]:443 ssl http2;
to
listen [IP]:443 ssl;
http2 on;
Save the configuration file and restart the Nginx server using the following command:
systemctl restart nginx.service