"DB Error: Syntax error" - after using false "find and merge duplicate" rule
Since you marked it as Reserved you cannot modify or delete it.  (It can be gotten rid of by directly modifying the database, but I think that is the only way.)

Your best bet is to ignore it, and create a new rule that you can use.  Just don't mark it as Reserved until you are sure that it is defined as you wish.  

Note that you can edit the name and usage type, so you could rename it "DO NOT USE" and make it a General rule.