Upon executing a :
 Alter Index [UI_myindex] On [MyDb].[dbo].[Mytable] ReOrganize

the following error message was returned :

Msg 2552, Sev 16, State 1, Line 1 : The index "UI_myindex" (partition 1) on table "Mytable" cannot be reorganized because page level locking is disabled. [SQLSTATE 42000]

I issued the following SQL Statement on the relevant database :

Select A.Name as InName,ob.Name as DBName from sys.indexes A
left outer join sys.objects ob on ob.object_id=A.Object_id
where allow_page_locks=0 and ob.type='U'

and indeed this index was the only index with  :allow_page_locks=0

I discovered a developer had added the index via the SSMS , which by default has page_locks turned off.
They were not even aware that this was their intention

The first thing I did was change their security details - retsricting them from making ad-hoc index changes
Secondly , I turned page - locking on for this index .

USE [MYDB]
GO
ALTER INDEX [myIndex] ON [dbo].[myTable] SET ( ALLOW_PAGE_LOCKS  = ON )
GO