More information needed to solve this problem! The prisoner probably broke rules and got caught. Maybe they beat someone up, got caught with contraband, etc. Possibly they are a higher security grade (e.g. Max, red shirt) and need a cell in a Max-only area. Get a psychologist right away - he will let you see their urgent needs, recent violations, and let you know if their cell is 'too high grade' (if it's not designated for their security grade in 'Deployment'; see the Mega-Guide for more info). He will also let you know if the prison is calm or ready to riot.