The reason is that there is no screen on which to display any windows. A crontab 'job' is running in its own little login. It's a command line -only session, hence nowhere to send graphical display output.
The 'no MTA' message is a different thing. The crontab task failed (nowhere to display graphic output), so crontab would like to send an error report. But there's no email service set up for the OS to use. MTA is Mail Transfer Agent, which on Linux is typically sonething like sendmail, but also (typically) is not configured and running by default. Adrian W. -- Next meeting: Online, Jitsi, Tuesday, 2022-07-05 20:00 Check to whom you are replying Meetings, mailing list, IRC, ... http://dorset.lug.org.uk New thread, don't hijack: mailto:email@example.com