New install of Ubuntu 18.04 MATE
I have been using LMDE2 MATE with no problems.
There is no Update Manager Indicator in the panel.
I followed the instructions for ubuntu 12 here but these did not work, though it did install a lot of other stuff 8 mg plus:
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install indicator-updatemanager
I ran:
update_indicator --with-preferences
which gives command not found.
Open the synaptic Package manager and search for update-manager there should be 3 files installed
- update-manager
- update-manager-core
- python3-update-manager
if not there install them
Thanks. All present and correct.
I often get a message like this:
MATE Indicator Applet has quit unexpectedly
But even when I go to details there is nothing I can copy and paste. (This is VERY frustrating! There are at least 30 lines in the details.) I always press send of course. 
Check .xsession-errors and see if there has been any crashes on startup like blueman - if there is uncheck those items in the session and startup.