I cant copy and paste data frames in ArcMap 10.5. What gives? I can copy but when I try to paste a duplicate in table of contents, it does not highlight the paste, this does not allow me to paste.
Look at this duplicate thread for some bug information and a workaround:
https://community.esri.com/thread/194681-cant-copy-and-paste-data-frame-in-arcmap-105
Direct link to the Bug and workaround BUG-000088466: Unable to copy and paste the data frame in ArcMap 10.3.1.
To provide an additional alternative:
You can activate the layout view

... and select the dataframe on your page, copy, and paste on the layout view.
I am afraid, it doesn't work for me in Layout View as well (10.3.1 - 10.5).