How to make file manager load in Drupal
After you install Flmngr module for Drupal, please make sure that you have enabled it. You can check this in the Extend section of the Drupal dashboard. If enabled, the Flmngr module will automatically enable itself for the Full HTML text fromat if you use CKEditor 4 or CKEditor 5.
Upload, Flmng, ImgPen buttons into the active toolbar to enable them. To do this, please go to the Configuration → Text formats section.If you still do not see new buttons in your CKEditor, first check if you are really using CKEditor. If yes, please go to the Configuration → Text formats section and see if there are "Flmngr" badges near the desired formats. Maybe you are trying to edit an article with the "Basic" text format, but the Flmngr file manager is only attached to the "Full HTML" format.
Or do you have a custom text format and there is no Flmngr badge near it? If so, please click on "Edit" next to it and enable Flmngr inside it.