If you do not have a file called Driver.szs, you can download it from https://avsys.xyz/MKW/Driver.szs

Open up Driver.szs in brawlbox, and scroll down to wr.brres is. Right click wr.brres and hit replace.
When the file selection opens, click the new wr.brres in this folder and then save.

Make sure that your Driver.szs is placed in your My Stuff folder after saving.