Spoiler Tags

I am having trouble using spoiler tags while editing my movie review.
I used the brackets ( [ , ] signs), but when I click save, this is what comes up.
Error
I used the brackets ( [ , ] signs), but when I click save, this is what comes up.
Error

You need to highlight the text and select the format block option (6th icon) in the editor and then select spoiler

This really helped! Thank you very much!