why am i getting the following issue with the ssl certificate i have installed on my virtualmin?
Pretty much exactly what the message says.
You have some elements on the page that are loaded via http (not https ) - so these are not secure. You should ensure all elements loaded within your https page are loaded using https not http.
In terms of which ciper you use, that’s determined by your config. If you want to modify your config to relatively standard configurations, have a look at https://mozilla.github.io/server-side-tls/ssl-config-generator/