Negative testing on paypal sandbox no more available?

It is available, but you have to go to https://developer.paypal.com/developer/accounts

This option is not available at https://www.sandbox.paypal.com

Also you may need to link your account if you already created an account in the sandbox: https://devblog.paypal.com/sandbox-account-linking/

You need to find a screen that looks like this and click Profile

Then the option is available.

In order to enable correctly Negative Testing:

1. Navigate to the Profile > Settings page of your merchant’s test Business account (see Managing your Sandbox accounts for details).
2. On the Profile tab, set Negative Testing to On.

In order to trigger some particular error please check at this link the updated guide for Testing API error handling routines: https://developer.paypal.com/webapps/developer/docs/classic/lifecycle/ug_sandbox/#errors

Use negative testing to trigger the following types of errors:

• Errors that result from calling a PayPal API.
• Errors related Virtual Terminal and the DoDirectPayment operation (verification and credit card validation errors).

You can force two types of API errors: those related to the transaction amount, and those not related to the amount.

Note: Negative testing is available only in the Sandbox—you cannot force or simulate error conditions in the live PayPal environment. Negative testing can be used with Classic PayPal API versions 2.4 and later.