Short method, open the normal template directly and make your modifications there. See Normal Template in Microsoft Word - How to Open

Chances are good that you have an Add-In that is preventing the routine saving of changes to the Normal template in the background.

Check on your Developer Tab and look for both kinds of Add-Ins. Turn them off and try saving changes to the defaults. If you figure out which one(s) cause the problem, consider uninstalling them.