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Why is it that I can't choose an accepted answer for this question? The "accepted answer" option does not show up.
Unsafe use of user-supplied GString:s in Groovy/Grails
According to the revision history you've previously had a bounty on the question and never accepted an answer. This is discussed and explained here.
It's a known "bug". It is a result of an unresolved bounty (you did not select a winner, and one was not auto selected since no post had 2 or more votes). You will no longer be able to select an answer on this question.