You have selected either an attachment or a note in the middle pane. Attachments and notes do not have full bibliographic metadata and cannot have other items attached to them, so you will not see the Info, Notes, Attachments, Tags, and Related tabs when you select them. Only “regular items”—with reference types such as books, journal articles, and newspaper articles—will show tabs in the right-hand pane.

Tags and related items can still be associated with attachments and notes using the [Related] and [Tags] buttons at the bottom of the right-most pane.

See the library items overview for more information.