ckeditor on Drupal 8 not working

I have migrated a Drupal7 site to both Drupal8 and Drupal9.
When I go to http://drupal8/admin/config/content/formats/manage/full_html?destination=/admin/config/content/formats
I get the following:
Toolbar Buttons
[[{"name":"Formatting","items":["Bold","Italic","Strike","Superscript","Subscript","-","RemoveFormat"]},{"name":"Linking","items":["DrupalLink","DrupalUnlink"]},{"name":"Lists","items":["BulletedList","NumberedList"]},{"name":"Media","items":["Blockquote","DrupalImage","Table","HorizontalRule"]},{"name":"Block Formatting","items":["Format"]},{"name":"Tools","items":["ShowBlocks","Source"]}]]

When I go to http://drupal9/admin/config/content/formats/manage/full_html?destination=/admin/config/content/formats
I get the following:
Toolbar Configuration
Move a button into the Active toolbar to enable it, or into the list of Available buttons to disable it. Buttons may be moved with the mouse or keyboard arrow keys. Toolbar group names are provided to support screen reader users. Empty toolbar groups will be removed upon save.
Available buttons

Active toolbar

Any ideas on what could be the problem?