a) Enable debugging to show the error: [https://www.wpbeginner.com/wp-tutorials/how-to-easily-enable-wordpress-debug-mode-to-fix-site-errors/])
b) Report the error in the Litestart support forum so the developers can look at, and hopefully fix the issue: [https://wordpress.org/support/plugin/wp-maintenance-mode/])
c) Disable debugging on the site.
d) Disable the plugin and wait for a fix to hopefully be pushed by the developers.
Or disable and delete the plugin, and search for another than provides the same functionality you are looking for and that does not throw a fatal error when you activate it.
You have two choices.
Either troubleshoot:
a) Enable debugging to show the error: [https://www.wpbeginner.com/wp-tutorials/how-to-easily-enable-wordpress-debug-mode-to-fix-site-errors/])
b) Report the error in the Litestart support forum so the developers can look at, and hopefully fix the issue: [https://wordpress.org/support/plugin/wp-maintenance-mode/])
c) Disable debugging on the site.
d) Disable the plugin and wait for a fix to hopefully be pushed by the developers.
Or disable and delete the plugin, and search for another than provides the same functionality you are looking for and that does not throw a fatal error when you activate it.