Hi Q,
Welcome to our community! 
Your problem sounds indeed odd. Did the layer have got another name before? Or could there have been a typo such as “⍽Climbing” or “Climbing⍽”? The spaces are characters, too. (In my example, ⍽ is supposed to be a placeholder for a space character to make it obvious what I mean.)
If the layer had another name, there might be or have been an internal problem in Unity. Maybe outdated data was still in the cache, maybe Unity used that data. In a browser, you would simply clear the cache. In Unity, that’s not possible. Restarting often fixes issues, and you could (while Unity is not running) delete the Temp and Library folders in your Assets folder if a weird problem persists.
Before experimenting around, make a backup of your project folder by duplicating it. Save the duplicated folder somewhere else, maybe on an USB stick. To save some free space, you could delete the Library and Temp folders in the backup folder.
If you don’t want to experiment around, use layer 8.
One more thing: The screenshot in the manual shows that the numbers of layers 3, 6 and 7 are greyed out. Is that also the case in your version of Unity? Unfortunately, the doc does not reveal any information on that, so the greyed out numbers might be related to your problem, or maybe they don’t.
See also:
- Forum User Guides : How to mark a topic as solved