Why "a pray" is incorrect—understanding proper usage

What kind of error is it?

Grammatical error due to misuse of a verb as a noun

Why do people make this mistake?

People may mistakenly use "a pray" due to confusion between the verb "pray" and the related noun "prayer".

What is correct?

The correct usage depends on the intended meaning. If referring to the act of praying, the correct form is "a prayer". If describing the action, "pray" remains a verb without an article.