Based on your screenshot, it looks like you're trying to add a bone to a skin, but the "Add to Skin" button might be disabled or not working as expected.
Here are a few things to check:
Make sure a skin is active:
The Add to Skin button is disabled if no skin is currently active. You need to have a skin selected and active in the tree before you can add bones to it.
Two ways to add bones to skins:
Select a skin first: Click on a skin in the tree, click the Add to Skin button in the tree properties, then click the bones you want to add to that skin.
Make a skin active, then select bones: Make a skin active, select the bone(s) you want to add, then click Add to Skin in the bone's properties.
Note about the root bone:
The root bone cannot be added to a skin - only child bones can be added.
Viewing bones in other skins:
If you're having trouble seeing bones that belong to other skins, you may need to uncheck the Hide skin bones and constraints option in the tree view settings. This will show all bones regardless of which skin they belong to.
If you're still having trouble, could you provide more details about what happens when you try to add the bone to the skin?
