I don’t see how Send would depend on Apache (nowhere is it listed as a dependency on that page, and they seem to provide a Docker image, as well). Why was Apache in the picture at all, if you’re happy with nginx?
You probably need to uninstall the Apache package. Webmin, and thus Virtualmin, detect Apache based on the existence of files found in the package. If you have those files, it’ll think that’s the web server you want to use (Apache is the default, so it’s checked for first, if it’s found, nginx is probably never considered an option).
You can also try Re-check Configuration. I’ve never tried converting a system from Apache to nginx, or switching back and forth, and it’s not a supported path. It’s bound to be pretty messy.