Tagging photos with keyword does not work?
Don't forget to open the keyword manager (Command K), or you cannot add keywords in the Info Window (Comand I).
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I'm not sure what you are saying. You can add a keyword to an image by just typing into the keyword section of the Info window, even if the Keyword Manager is not open. If you type a word that is not already one of your keywords, it will create a new one. If you start typing a word already in the keyword list, then it will give a list of keywords that start that way. This all works just fine without opening Keyword Manager.
You do have to have the Keyword Manger window (command-k) open to use shortcuts. (Frankly, this is a hassle!) If you set "c" as a shortcut for the keyword "Castle," for instance, then you can scan through pictures and hit "c" when you see a castle, and it becomes a keyword for that picture. But the Keyword Manager must be open for this.
Find photos by keyword in Photos on Mac – Apple Support (UK)