I am following the Tutorial of the Wiki to make this Vault74a.

Though, since I use FalloutNV, I copied the Vault3a cell instead, called my own Vault "Vault66a" and proceed with the Tutorial like that.

Now I am at the point of the Tutorial where I am told to delete all the NavMesches using "Remove Cell Navmeshes". And it simply does not work. I already have loaded another cell and loaded mine again. I restarted Geck, I reloaded everything... tried it multiple times. With no success.

What can I still do?

Hmm, for some reason now it worked. Even though I simply did the whole thing again. Doing exactly the same as before.
Maybe I made a mistake somewhere before, or it was glitching.