Elementor stores templates, kits and global widgets in a custom post type named elementor_library. The plugin registers this type during its initialization. When the type exists, Elementor loads the library editor and applies saved designs.
The error appears when WordPress does not recognize elementor_library. Reasons include missing registration, a capability that the current user lacks, or a filter that removes the type from the public list. Conflicting plugins or corrupted core files may stop the registration hook from running.
Impact reaches every part of the builder. Templates disappear from the library, the editor refuses to open saved items, and pages revert to the default WordPress editor. Import or export of kits fails with the same message.