Is the word lunch a pronoun?

No, the Word 'lunch' is a noun and a verb.

The noun 'lunch' is a Word for a meal eaten in the middle of a day.

The verb to 'lunch' means to eat a meal in the middle of a day.

Examples:

Our lunch is ready. (noun, subject of the sentence)

She likes to lunch at the cafe around the corner. (verb)

A pronoun is a Word that takes the place of a noun in a sentence.

Example: They served lunch at the meeting. It was soup and Sandwiches. (the pronoun 'it' takes the place of the noun 'lunch' in the second sentence)