Ubuntu 18.04 LTS was released in 2018-April (thus the 18.04 using the year.month format used), with 5 years of standard supported life; ie. it's about to reach it's EOSS in mere days.
I posted notice(s) of that at Lubuntu's site and you'll find the same here too for Ubuntu MATE 18.04.
The oldest supported release for Ubuntu flavors currently is Ubuntu 20.04 LTS, and that's about to reach EOL too (Ubuntu-MATE's warning can be found here).
You may have just met the Ubuntu system, but it's still not on-topic for this site as I see it sorry.
FYI: Ubuntu 18.04 LTS' standard supported life received an extension awhile back, instead of EOSS in April 2023 being the end of five years, Canonical have elected to extend life until 31-May-2023; but given we're talking about flavors here anyway, that doesn't make any difference as I see it here.