If the user is a developer or advanced user:

Most of the time, the files you need are already on your computer, just not where the app expects them. Go to the folder where the application is installed.

But that’s source. Better: Use pre-built from:

The error message is a common headache for gamers and software users alike. Often, people search for a "Qt platform plugin download repack" to fix this, but downloading a random "repack" of DLL files can be risky.