I have been having the same issue and did not want to start a new project to georeference. I went to the "add commands" ribbon and added scale (search by name or under the georeference prepare tool options) and it added it to the ribbon. I selected the new tool from the ribbon and then also went back to the georeference tab and selected the scale tool; and the scale tool worked. I zoomed out to the whole raster and was able to grab the corners finally. This solution is not logical but it worked for me.
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