What makes a Roman Numeral invalid?

Answer 1

When it doesn't comply with the rules governing the Roman numeral system.

Answer 2

A Roman numeral can be considered invalid due to several reasons. These include using a repetition of the same numeral more than three times in succession (e.g., IIII instead of IV), writing a numeral in a descending order (e.g., XCX instead of C), or using a numeral that is not part of the standard Roman numeral system (e.g., A instead of I).