As of January 20, 2022 Ubuntu MATE 21.04 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.
See which version you are running by opening the System Monitor and checking the System tab.
The upgrade path will be to the current interim release, 21.10, supported until July 2022. To upgrade, open Software Updater and follow the steps.
Or alternately, via the Terminal: