It's because either or both of the queries are returning more than one ID.

SELECT ID FROM MyCTE WHERE FullContent = 'Authorized Privileges:'

SELECT ID FROM MyCTE WHERE FullContent = 'Default Privileges:'

Do you have, in your data more than one records with FullContent = 'Authorized Privileges:' or FullContent = 'Default Privileges:'?

If yes, you must get only one ID from the above queries. Use distinct/max/min/ Top 1 as per your requirement.