A family of Microsoft word processing software products for creating web, email, and print documents.
Since you are not allowed to (re-)assign the shortcut successfully, try opening the Normal template as a document, assign the shortcut, save and close the file.
The quickest way to open Normal is by using a quick macro "trick": From within Word, press Alt+F11; the Visual Basic Editor displays. Press Ctrl+G to activate the Immediate window. Type
NormalTemplate.OpenAsDocument
and press Enter. Normal opens in a separate window (and you should now close the Visual Basic Editor). Make the changes, as discussed above. Save and close when you are done.
The next step is to figure out what is preventing you from saving to Normal. You can troubleshoot poorly designed add-ins by following the manual steps in the article at https://support.microsoft.com/en-us/kb/921541.