Failure to update/upgrade

I’ve downloaded and imaged to new USB sticks (Pi modified to boot from USB) the ‘latest’(? March 2018) build from http://dvswitch.org/files/ASL_Images/Raspberry_Pi/Stretch/ and note a couple of ‘concerns’ - apt-get update, apt-get upgrade and trying to install Allmon2 all result in an error: “E: unable to locate package…”

In one test case I pushed through yesterday I did a source update and was able to get a couple of things installed - Apache2 and php - manually. But otherwise, worked at a repeater site over the weekend I’m left with an incomplete but functioning repeater system on-line and linked, no Allmon2, and trying to create backup sticks to have in toolbag and on-site… troublesome. I’m reverting to cloning a working ‘reference’ stick as future backup.

Is there something obvious/stupid I’m doing wrong?