Where did you download the reengagement plugin from? The version on the Moodle plugins directory is not listed as being compatible with Moodle 3.10. If you installed it from the Moodle plugin directory, you should uninstall the plugin. Are you able to access this page directly?
https://<site>/admin/plugins.php
If you're able to access it, attempt to uninstall the plugin. Once uninstalled, make sure the plugin files are deleted from the server.
A newer version of the plugin seems to be available from Catalyst IT on Github:
https://github.com/catalyst/moodle-mod_reengagement
It says that the 'main' branch is compatible with Moodle 3.9 and higher. This is the default branch that displays on the Github website. If you click 'Code' -> 'Download ZIP' you can download this version.
https://github.com/catalyst/moodle-mod_reengagement
It says that the 'main' branch is compatible with Moodle 3.9 and higher. This is the default branch that displays on the Github website. If you click 'Code' -> 'Download ZIP' you can download this version.