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The "head" of that retainer has been broken off. Take it off carefully, with a trim tool or otherwise so as to not damage bumper, and replace it with a new one when you install the bumper.
Get a small ice pick or something that will fit that hole in the centre of the rivet and push it in. That will straighten the shaft of the rivet and you'll feel it coming slightly loose.Then get a screw driver or a trim tool to pry it out.