My keywords editor button is not selectable

I have spent the last few days applying keywords to some new footage I recently imported into my browser. I was nearly finished when I closed the library I was working on. When I opened the library again the next day to continue applying keywords to the footage, the keywords editing button was not selectable.

I am selecting in and out points of a clip in my browser, like always, but then when I go open the keyword editor the key button is not selectable. I can still drag and drop clips to existing keyword collections, but I can't create new ones.

I have tried closing the library and even restarting my computer, but the issue still exists. The key button is there, but greyed out and when I click on it nothing happens.

Why is this happening? And, how can I make my keyword editor button selectable again so that I can create new keyword collections?

Thanks in advance for your help.