Sending credit card numbers directly to the Stripe API is generally unsafe. We suggest you use test tokens that map to the test card you are using

Why am I encountering errors when using the test card number "4242424242424242" on my newly created Stripe account, even though I've added the required Secret key to my Node Express routes? and also I have tested with a similar card number available in the docs. but it give error
Sending credit card numbers directly to the Stripe API is generally unsafe. We suggest you use test tokens that map to the test card you are using.

But it works in my old Secret key.

Stripe flags this in their documentation: https://stripe.com/docs/testing?testing-method=card-numbers#test-code

> We don’t recommend using card numbers directly in API calls or
> server-side code, even in test mode.

I understand you're working in test mode right now but think about the production scenario: unless you're PCI compliant, your server-side code should never have access to a customer's raw card number. In production, you'll likely use Stripe Checkout or the PaymentElement to collect a customer's card number. The card number will be shared directly with Stripe, and Stripe will create a PaymentMethod ID to represent that real card. The PaymentMethod IDs in Stripe's testing doc are what you should use server side.

I'm having this exact same issue (I'm creating TokenCardOptions using the "4242424242424242" card), but I only get this exception while using my day-old secret key. When I use my coworker's stripe API secret key (two months old), this exception isn't thrown.

        TokenCreateOptions tokenOptionsCredit = new TokenCreateOptions
        {
            Card = new TokenCardOptions
            {
                
                Name = "John Smith",
                Number = "4242424242424242",
                ExpYear = "2042",
                ExpMonth = "12",
                Cvc = "555"
            }
        };


        TokenService tokenService = new TokenService();

        // Create new Stripe Token
        Token stripeTokenCredit = tokenService.Create(tokenOptionsCredit);

I agree with LauraT's answer; my test account isn't PCI compliant, so I shouldn't be able to use a raw card number. But my coworker's test account isn't PCI compliant either, so I'm still curious as to why this works with his secret key, but not mine.

This is not recommended in Stripe's newest API version.

Instead do

// https://stripe.com/docs/api/cards/create
StripeConfiguration.ApiKey = "YourKey";

var options = new CardCreateOptions
{
  Source = "tok_visa_debit", //https://stripe.com/docs/testing?testing-method=tokens
};
var service = new CardService();
service.Create("YourStripeUserId", options);

// Example: this method will create new customer with a payment card
public async Task<Customer> AddStripeCustomerAsync(Customer customer, CancellationToken ct)
{
    // Set Customer options - We need to create the customer first as we will need the id 
    CustomerCreateOptions customerOptions = new CustomerCreateOptions
    {
        Name = customer.Name,
        Email = customer.Email,
    };

    // Create customer at Stripe
    Customer stripeCustomer = await _customerService.CreateAsync(customerOptions, null, ct);

    // Set Stripe Token options based on customer data
    var cardTokenOptions = new CardCreateOptions
    {
        Source = "tok_visa_debit",
    };

    // Create new Card Token
    var cardToken = await _cardService.CreateAsync(stripeCustomer.Id.ToString(), cardTokenOptions, null, ct);

    // Return the created customer at Stripe (good for testing, to see if the request gone through)
    return new Customer(stripeCustomer.Name, stripeCustomer.Email, stripeCustomer.Id);

    /*In the code above we are creating a new customer and the customer's credit card at Stripe through the API. 
     * We start off by creating a new token object that contains the customer's credit card details.*/
}

Card created against user

> But my coworker's test account isn't PCI compliant either, so I'm
> still curious as to why this works with his secret key, but not mine.

This error is a feature of new Stripe accounts following changes to how you can enable Server-side payment method collection :
https://stripe.com/docs/security/guide#:~:text=If%20you%20continue%20to%20send,implement%20to%20remain%20PCI%20compliant.
This would be normal if your coworker's Stripe account is older than yours.

You'll have to contact Stripe support and submit a SAQ D form before this error can be bypassed on your account.

Or, as suggested above, use test tokens, or don't pass card information server-side, but client-side using functions documented here:
https://stripe.com/docs/js