I think "don't" is accidentally omitted in this error message.
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However, I suggest that we abolish the 'retag' button and this error message, and allow edits which only affect the tags to go through if the user has the retag privilege. The 'edit' button is more obvious (as xanadont also experienced) and I'm trained to use that one as it's the only option available to me on all the other sites I participate on. The 'retag' button is an MSO-only thing for me.
I understand the argument that the post should be edited completely, but we have the retag privilege for a reason - combine it with the edit option, and this error message will be unnecessary.
As ChrisF correctly noticed, you were trying to perform "full edit" of the question, which for users with less than 2K reputation means suggesting an edit. Suggested edit must include "real" change in the post itself (body or title) so just changing the tags results in what you got.
However I agree that adding "To change tags only, please click the retag link" or something along those words will makes things more clear.