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entity type does not exist. in Drupal\Core\Entity\EntityTypeManager->getDefinition()

Go to your content type
manage display
Check those 3 checkboxes
1-Panelize this view mode
2-Allow each content item to have its display customized
3-Allow users to select which display to use
SAVE
click on Add a new Panelizer default display
next next ( you know what to do ) and save.
GO TO : Manage form display
Let the paneizer field show up on the node edit form
save
go to your node and choose your NEW display ( which is not the default )
save and publish your node

source: https://www.drupal.org/project/panels/issues/2875000