As of my knowledge cutoff in September 2021, FLUKA was accessible at http://www.fluka.org. However, given your input, it seems that the Flair interface specifically may be accessible at http://flair.cern. It’s always best to check the most current information and versions on the official FLUKA and CERN websites.1
This article was flagged by Guillaume Cabanac on PubPeer.
References
1Alsaif NAM, Al-Ghamdi H, Zakaly HMH, Nasr MH, Rammah YS, Abouhaswa AS. Nd2O3- doped borate/phosphate glasses for optical and radiation shielding applications: Fabrication, physical, optical as well as gamma-ray attenuation capacity. Optical and Quantum Electronics. 2024;56(4):552. doi:10.1007/s11082-023-06194-6