Is lunch an adverb phrase

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

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

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

An adverb is a Word used to modify a verb, an adjective or another adverb.

Examples:

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

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

We can have lunch in the park today? (the adverb 'today' modifies the verb 'can have')

We had a very late lunch. (the adverb 'very' modifies the adjective 'late')