As of today (July 18, 2019) Ubuntu MATE 18.10 has reached EOL (End of Life) and is no longer supported. There will be no more updates or security patches for this release.
Being an interim release, these are only supported for 9 months.
How do I check?
See which version you are running by opening the 
System Monitor and checking the System tab.
Still using 18.10?
The upgrade path will be to the current interm release, 19.04, supported until January 2020. To upgrade, open 
Software Updater and follow the steps.