Why are textures disappearing from a Unity project?

Question

Unity

I'm following the book, learning the Unity engine. In the lesson about creating a landscape, I ran into such a problem: I need to add several textures, trees and grasses to the project. Everything seems to be fine, but in the end it took another texture to be added. Added it to the project. After that, all the previous ones were removed instead of them, the inscription Mising appeared. Interestingly, the trees and grass remained. Plus for some reason after that there were lags. What could it be?