I want to buy my wife Apple Music for her iPhone. Can I do it from my phone?

You can send iTunes gift Cards from your phone. You would have to select the denomination.

https://www.apple.com/shop/gift-cards

“App Store & iTunes Gift Cards are solely for the purchase of goods and services on the iTunes Store, Apple Books, Apple Music, and the App Store“

I can’t see a Card that’s just for Apple Music, it would be for her to spend at her discretion on iTunes, from her device she signs in with her Apple ID to use it. Depending on how much you gave her, once the funds ran out, she would be charged to keep up the subscription.

I think some Brick & Mortar stores might sell Apple Music Gift Cards,

*Don’t get the Apple Store gift card - that’s to buy physical products on Apple’s website (located on the bottom of the page)

Correction:

i meant the Apple Store gift card is only for use in physical Apple Stores or their online store. It’s not the one you wanted. anyways.