Why is my document window not active?

So I’m 1000 words into a 9000 word paper - the final paper for my masters - and I decide to update my version of Word when prompted. 

Now when going to insert an End Note reference into the footer (I use a variation of Oxfood footnotes), EN gives me the following message:

“This command is not available because a document window is not active.”

I have googled this and cannot find a simple solution.

As always, any and all help given greatly appreciated!

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