It took you 1 second to do this two processes. I will assume that the buildings file is empty too or you did populate it before you make this attempt.
You must have four new files in the WoW directory after everything is done properly (dbc, mmaps, vmaps, buildings.)
first check if buildings file is empty or not.
If empty or dose not exist you must create it. just create New file then rename to buildings inside your WoW directory.
then u must specify the right path to your DATA file which is inside your World Of Warcraft directory to the vmapExtractor.exe
How to:
open your WoW directory, find the DATA file right click and open properties in the Target: box copy the text.
Then go back to your WoW directory just right click on your vmapExtractor.exe and create shortcut to your desktop.
Then go to your desktop right click on the shortcut open the properties in the Target: (there is already text there do not delete or change it just add at the end of it) -d (and right click, then paste) in the target path it should look something like this:
example: "C:\\Games\\Worold of Warcraft\\vmapExtractor.exe" -d "C:\\Games\\World of Warcraft\\DATA"
apply then close/OK.
Now run vmapExtractor.exe from the shortcut. You must wait to finish the extraction.
Now you must have buildings file full.
Go to your WoW directory again find the vmap_assembler.exe
right click on it and run Git Bash or open with command promote and type: vmap_assembler.exe buildings vmaps
press enter.
Now wait the assembler to finish.
...you should now have all four files full just copy them were you have compiled your server.