That's the upgrade notice for UpgradeMODX itself. The Upgrade widget shows on the dashboard you see when you log in, though your version of MODX is about 8 years old and doesn't have a dashboard. You should have a resource called "UpgradeMODX" If you view it, you should see a version of the widget.
See the box on using UGM with very old versions of MODX about 1/3 of the way down the page here: https://bobsguides.com/upgrade-modx-package.html
Change the versionsToShow property of the UpgradeMODX Widget snippet to 99 so it will show the older versions. You may have to manually delete the files in the core/cache directory to get that change to take effect.
You'll want to upgrade to 2.1.0, 2.2.0, 2.3.0, 2.4.0, 2.5.0, and 2.6.0 before going to the current version to avoid missing important changes to the database. After each upgrade, check Package Manager for updates to extras, otherwise, an outdated extra may prevent setup from running.
If using the resource doesn't work on a version that old, you may have to resort to the traditional method of upgrading: https://bobsguides.com/modx-upgrade-faq.html
Considering the volume of attacks on MODX in the last week, before you start upgrading, I would recommend moving your core directory above the web root and renaming the connectors directory as described here: https://bobsguides.com/blog.html/2015/05/05/hardening-your-modx-site/
. If you have the gallery extra, remove it immediately and download the current version when you get to MODX 2.6.5.
[ed. note: BobRay last edited this post 6 months, 3 weeks ago.]