Map making error

Hey all, just recently I decided I wanted to make a map. (I'm a total newb btw) So I googled for some tutorials and found this one:
But when I try to do what he does at around 20:00 where he starts placing the dead trees, I get an error saying "Failed to create resource (is the file missing?) Levels/(Name)/(name)3.forest" The game then immediately crashes after. I noticed there was already a file named (name).forest, so I duplicated it it and named it (name3.forest). The same problem occurred but saying ""Failed to create resource (is the file missing?) Levels/(Name)/(name)4.forest" 4 instead of 3. The game proceeded to crash. Why is this happening? All the files are renamed, and everything else worked so far.
If you need a log or something, I don't know how to do that, but I have a screenshot.