You need to hold the nut from the back while you tighten the bolt. You might want to loosen it and remove it and check the thread then reinstall. They work bear and don't (usually) need to be held if the outside of the nut and the recess on the chainring are dry, while the thread, and bolt side are oiled.
I use abrasive paste (lapping compound) under the rim of the nut to improve traction and never need to hold it making the job easier.
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