Assigning Keyboard Shortcuts in Microsoft Word 2007-2021 (365)
I have no problem in Word 10 or Word 365 assigning Alt+J.

Because of the Alt+ tooltips to the ribbon, like Stefan, I avoid plain Alt+ shortcuts.

If you are assigning shortcuts to multiple macros, see the section in the above-linked page about using prefixes. I suspect that you may have already assigned Alt+J as a prefix. If you had, you would be seeing "Alt+J," in the box for the shortcut. It is waiting for another keystroke.


If you absolutely need this, check if you can assign it when you are running Word in safe mode. (This is a diagnostic step, not a way to assign the shortcut. Let us know the result.)
Open Office apps in safe mode on a Windows PC - Microsoft Office