A singular blunder: why "a criteria" doesn't make the cut (hint: it's "criterion")

What kind of error is it?

It is a grammatical error, specifically a singular-plural mismatch.

Why do people make this mistake?

People often make this error because "criteria" is commonly misperceived as a singular noun when it is actually the plural form of "criterion".

What is correct?

The correct singular form of "criteria" is "criterion". Use "criterion" when referring to a single standard or principle, and "criteria" when referring to multiple.