Incorrect or missing article before a noun.
Use 'A/An' for unspecified singular countable nouns. Use 'The' for specific/known nouns. Omit articles for general plural or uncountable nouns.
I saw dog in park.
I saw A dog in THE park.
Singular countable nouns (student, car, apple) can NEVER stand alone. They need a/an/the/my/this.