Webmin Unable to correct problems, you have held broken packages

I tried to install webmin to my 18.04 erver. i folloved DigitalOcean 's method.

When i did

sudo apt install webmin

The outpu was like:

   Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

The following information may help to resolve the situation:

The following packages have unmet dependencies:
 webmin : Depends: libauthen-pam-perl but it is not installable
          Depends: apt-show-versions but it is not installable
E: Unable to correct problems, you have held broken packages.

Thank you all your answes,
Peter