The "insert_image" plugin does not exist

I assume you tried to switch from Insert module version 8.x-1.x to 8.x-2.x.

8.x-1.x used custom widgets like the "insert_image" widget. These have been removed in version 8.x-2.x, see also the release notes of the latest release (section "Updating from 8.x-1.x"): https://www.drupal.org/project/insert/releases/8.x-2.0-beta3 →

As per https://www.drupal.org/project/insert/issues/2976986 → , the Insert module would first need to be set up on the relevant fields again, then it should work and you should be able access the form display management page. Unfortunately, there is no smoother update due to that basic architectural change.

Feel free to reopen if the issue is still relevant.