What is one life one chance in Latin?

Occasio una = One Chance

ADDED:

'Fors' does mean 'chance', but it means chance in the sense of luck or fortune in general - like 'a game of chance'. It is a feminine noun, so the modifier cannot be 'unicus' - that's a masculine form.

'Occasio' means chance in the sense of a single happening - an opportunity or chance at something. It is also a feminine noun, and its adjective has the correct ending.