Installing CATIA V5 R21 on Windows 11 requires specific compatibility adjustments because the software predates the operating system by over a decade. While not officially supported by Dassault Systèmes, you can successfully run it by using Windows 7 Compatibility Mode and ensuring the correct environment variables are set.
The primary hurdle when installing legacy CAD software on Windows 11 is the operating system's advanced security and driver signature requirements. Before beginning the installation, the environment must be "downgraded" in terms of its restrictiveness. install+catia+v5+r21+on+windows+11+updated