==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): last free version
- Deployed with iRedMail Easy or the downloadable installer? download
- Linux/BSD distribution name and version: ubuntu 20.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): - 
- Web server (Apache or Nginx): -
- Manage mail accounts with iRedAdmin-Pro? no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I was following official steps to setup the free version and I ended with "dialog command not found". I has to install it: apt -y install dialog

After that I was able to run the wizard. I think iredmail setup needs to check this package or add in system preparation in the official setup doc.