Re:"Bank got debited but its not recorded in my history on apple service, nor did i get the product
I was buying 400 robux, so after i paid it i got charged my bank account got debited."
If the purchase was made through the App Store or the App Store’s In-App Purchase system, it would show in your purchase history: See :View your purchase history for the App Store and other Apple media services - Apple Support
Apple Support have an article online that can help understand just how in-app purchases work.
Buy additional app features with in-app purchases and subscriptions - Apple Support
As your bank account was debited, you may still have access to the product you purchased.
See what to do: If you can't find or use your in-app purchases - Apple Support
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Re: "When i returned to the app it showed an error has occured"
It seems likely any money involved is with those selling the robux, rather than with Apple:
About purchases not billed through the App Store or the App Store’s In-App Purchase system - Apple Support
Eg: says:
"If you paid for an app, a purchase within an app, or a subscription outside of Apple’s ecosystem, contact the developer for billing support or a refund. Apple is unable to provide support for these purchases."
There is more info. in the linked article, in order to help.
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Re: "I dont know how to reach out to apple or raise a complaint"
Seems Apple may have merely passed a transaction on for you, much as a bank would do ?
However, if needed:
To report a concern about an app, you must provide the URL of the app.
Find the URL for content to report a concern - Apple Support
Then: How to report a problem
https://reportaproblem.apple.com/?s=6
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For Info: / support / contact options etc. Apple's Billing and Subscriptions Support website:
Subscriptions and Billing - Official Apple Support