Like many of you, I have been having this problem. An earlier post mentioned it could be because of a service running in the background. I use Dropbox. So, I paused Dropbox from indexing and syncing. Suddenly after re-starting ArcMap I was able to make the edits to a shapefile of zip code polygons that kept giving me this same error. Try pausing Dropbox and see if this works for you.
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