Final answer:
In Roman numerals, the digit that does not exist is zero (0).
Step-by-step explanation:
In Roman numerals, the digit that does not exist is zero (0). Roman numerals are based on a combination of letters, such as I, V, X, L, C, D, and M, to represent numbers. However, there is no specific symbol for zero in the Roman numeral system. For example, the number 10 is represented as 'X' and 100 is represented as 'C', but there is no symbol to represent zero.