It looks like you mixed tabs and spaces (just look at your question in "edit" view to see the issue). While your editor views the code evenly indented, the Compiler seems to have a different interpretation how wide a tab should be, resulting in the writeFile destPath contents line being additionally indented. So the source is interpreted like this:
...
putStrLn ("Copying from " ++ srcPath ++ " to " ++ destPath ++ "...")
contents <- readFile srcPath writeFile destPath contents
putStrLn "Finished"
In this interpretation of the source code contents is used before it is created, so you get a compiler error.
To avoid these kind of errors best don't use tabs, or at least take additional care that you use them consistently.
