Does anyone know how to install virtualmin on dietpi?

if you run an empty system, you should be able to run installation as described on Virtualmin web site.

Already have webmin installed will it cause any issues or remove webmin first?

I don’t know. Something you need to ask the developer of Virtualmin. You could simply give it a try.

Says your OS is not supported by this installer

According the vendor you would need to run x86 or amd64. I tested it on Raspberry OS 64 (arm64) and it was working as well

  1. wget
  2. sudo nano
  3. Find the text “get_distro”
  4. Add in os_type=“debian” after get_distro
log_debug "Operating system name: $os_real"
log_debug "Operating system version: $os_version"
log_debug "Operating system type: $os_type"
log_debug "Operating system major: $os_major_version"

What this does is, it will force the script to recognize the host as a debian operating type.