It is not recommended to store credit card information directly on your website, as it can lead to security issues and can make your website a target for hackers. Instead, it is recommended to use a secure payment gateway like PayPal or Stripe that is designed to handle the storage of sensitive payment information.
If your client still wants to proceed with storing credit card information on their website, you will need to ensure that you are complying with all relevant security standards and regulations. Additionally, you will need to ensure that your custom plugin is integrated properly with Magento and that it is not causing any conflicts or errors with the payment information page.
To troubleshoot the 404 bad request error, you should check the Magento error logs and server logs to see if there are any error messages or indications of what might be causing the issue. You may also want to check the code for your custom plugin to ensure that it is properly configured and that it is not causing any conflicts with other modules or extensions.
However, it is highly recommended to discuss the risks and concerns associated with storing credit card information directly on the website with your client and advise them to use a secure payment gateway instead.