Method 2: Download a Repack/Missing Plugin (Third-Party Apps)
https://download.qt.io/archive/qt/5.15/5.15.2/qtbase/ (if available)
This error is the number one reason developers seek out platform plugin repacks. Seeing it is a rite of passage for anyone new to Qt. Here is a breakdown of why it happens and what it looks like.
Ensure qwindows.dll is located inside that platforms folder. Method 2: Set the System Environment Variable