Shortcut cannot be set with Merge Cells in Excel

Dear all,

I am using excel for listing all literature for my research, and want to use short keys as many as possible for efficiency. I'd like to have a shortcut with the function of 'Merge Cells' or 'Merge & Center', but the setting through System Preference > Keyboard> Shortcuts> App Shortcuts> + does not work. I wondered if it is the problem of not having 'exact name of the menu command', but as far as I know, Merge Cells is the exact name.

Of course, I turned off and on both the excel programme and the computer. Didn't work.

Had anybody this problem and somehow solved it out? I appreciate your help!