Why can’t I download apps it say I need to change payment method and the app is free

On mobile device go to Settings > iTunes & App Store > Apple ID and check your payment information is current. You need to do this even if you require something which is free.

Refer to this - https://support.apple.com/en-in/HT203005

Check the following

1. The payment method that you are adding is of the same country as that of App Store Country on your iPhone
2. You have valid payment plan / debit / credit card from an authorised bank from the same country
3. Payment methods that you can use with your Apple ID - Apple Support

If you can't choose None when you edit your Apple ID payment information

1. If you have an unpaid balance or payment due
2. If you have a subscription that automatically renews
3. If you're using the iTunes Store or App Store for the first time
4. If you are not in the country or region you’re creating an Apple ID for
5. If you changed your country or region
6. If you're a part of Family Sharing