Broken plugin

Try some basics first.

1 – Backup your installation. Not trying to frighten you, this is always a good first step.

https://codex.www.ads-software.com/WordPress_Backups
https://codex.www.ads-software.com/Backing_Up_Your_Database
https://codex.www.ads-software.com/Restoring_Your_Database_From_Backup

2 – Remove that plugin completely. Make sure the file directory is removed wp-content/plugins/post_list_widget

3 – Re-install again from the source.
https://www.ads-software.com/extend/plugins/post-list-widget/

If that doesn’t work, try de-activating all of your existing plugins and then re-activating just that one. This is just to see if another plugin is causing a conflict.