Why we say "two million" — not "two millions"

What kind of error is it?

It is a grammatical error related to the incorrect pluralization of a quantifier followed by a count noun.

Why do people make this mistake?

People often add "s" to "million" because, in general English usage, we pluralize nouns when talking about more than one. However, numbers like "million", "billion", or "thousand" act as quantifiers and remain singular when preceded by a number. The error comes from overgeneralizing the pluralization rule.

What is correct?

When stating an exact number, English uses the singular form: "two million", "three million", etc. The plural form "millions" is only used when talking about an unspecified, large quantity (for example, "millions of people").