Assuming your are using Rasbian or other flavor of unix on the Pi, you would need to change the folder permissions to allow the user the Nukkit Java process is running under to have read/write on the folder.
There are many different ways to do this, but if you are running under your own user the easiest way is to set the folder as yours using
Code:
sudo chown -R yourusername /path/to/nukkit/folder
Then, make sure you have read and write privileges
Code:
sudo chmod -R u+rw /path/to/nukkit/folder