Can't update usermin

I’m running Linux Mint 19.3 64bit and trying to install usermin 1.791

Preparing to unpack .../usermin_1.791_all.deb ...
Port 20000 is already in use
dpkg: error processing archive /home/xxx/Downloads/usermin_1.791_all.deb (--install):
 new usermin package pre-installation script subprocess returned error exit status 2
Errors were encountered while processing:

aptitude search usermin returns nothing.

nmap says 20000/tcp open dnp

None of the files provided by the Debian install package are on my system.

http://localhost:20000 results in the following…