What is happening, when you try to add a keyword?
And how are you adding keywords? Are you using the Keyword Manager panel or are you trying to type into the Info panel?
If the Info panel should be unresponsive or losing the focus, try to close all other apps, then restart Photos. Photos on Big Sur is sometimes not responsive after importing new photos, while it is still indexing them and adding them to the library. After importing a large batch of Photos, give it some time to add them them to the library and wait, until to new photos have appeared in the Imports, Recents, and All Photos. Only once you are seeing them in all albums, start working with them.