I want to verify my account, but Instagram does not complete the payment process, why?

Hello Olivia_brahim,

Thank you for reaching out to Apple Support Communities.

Since you are trying to verify the account of a 3rd party app or service it may be best to contact them directly.

These steps will guide you on contacting them:

"Contact the developer of an iOS, iPadOS, or watchOS app

1. Open the App Store on your iPhone or iPad.
2. Search for the app, then tap it. If you can't find it, tap the account button  or your photo at the top of the screen, tap Purchased, tap "Not on this [device]," then tap the app.
3. The app's product page appears. Scroll to the Ratings & Reviews section, then tap App Support. If you don't see App Support, make sure that you're signed in with the correct Apple ID."

Additional details can be found here: How to contact an app developer

They will take a closer look and check on available options.

Cheers!