Can’t fix my billing info to redownload messenger.

but it’s saying I already have that card in and that’s the card they keep declining. I don’t have money on the card right now

If you see a message that says "Your your payment method is declined in the App Store and iTunes", there is a problem with your payment method on your account.

See -> If your payment method is declined in the App Store or iTunes Store:

If your payment method is declined in the App Store or iTunes Store - Apple Support

Scroll to:

• How to complete your purchase if a payment failed or was declined
• If you still can't make a purchase
• Still need help? Contact Apple Support.

but why should I need it for a free app?

If you want to make purchases or update / download apps, including free apps, you need to have a payment method on file.

If you don't have a credit card or don't want to use a credit card, you can also use your Apple ID balance as payment method, see -> Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

See also: 

What you can buy with your Apple Gift Card or Apple ID balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

When redeeming a gift card, (1) the amount will be credited to your Apple ID balance and (2) then you need to set your Apple ID balance as your default payment.

(1) See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

(2) See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

Also read the articles below ⤵️

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

• You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.

About Gift Card Scams -> About Gift Card Scams - Official Apple Support

If you can't make purchases or update apps, including free apps, you need to update your payment method or add a new one. 

• If you want to change your card, you need to add first the new card, then remove the old card.
• or if you use your Apple ID balance, set your Apple ID balance as your default payment.

To verify your payment information you need to -> Sign in to your Apple ID account page and edit / update your Apple ID details https://appleid.apple.com                                                     

On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

Click here for more support and assistance.