SQL Server Management Studio won't let me add an index to a table
Solution: Close your table designers and database diagrams and try again. If that doesn't help, close all windows in Management Studio.
Cause: The "New Index" option gets disabled when the table is schema-locked by the designer window.
It could be a rights issue, or perhaps you've become disconnected.Try using code to add the index; that may resolve your issue, or report a more meaningful exception for you to work from:
create index ix_MyTable_Column1on dbo.MyTable(Column1 asc)
Close the table if opened in the designer. Right click on Indexes for the table and select Rebuild All. This will fix it...