Hello Juan Pablo,
I'm the Integrations Unit supervisor with 2Checkout. We believe the issue that you are encountering may be related to the Secret Word in your 2Checkout account not matching the Secret Word set in your ZenCart admin. We would first recommend that you review the recommended integration instructions linked to below:
https://www.2checkout.com/shopping-cart/zen-cart
We would then recommend to ensure the Secret Word that you have set in your 2Checkout admin matches the Secret Word entered in your ZenCart Admin.
The Secret Word in your 2Checkout account can be accessed by clicking the Account tab and then the Site Management sub tab. On the Site Management screen the Secret Word field is located near the bottom of the Checkout Options column. The Secret Word is set to a default value of 'tango' and we highly recommend to change the value stored in your 2Checkout account and then ensuring that the same value is entered and saved in your ZenCart admin. After you update the Secret Word in your 2Checkout account please be certain to click the orange Save Changes button located in the bottom right corner of the Site Management page.
I hope this information helps to resolve the issue that you had encountered. Please feel free to contact 2Checkout if you have any other questions or concerns.
Thanks,
Rick Saunders| 2Checkout
Integrations Unit Supervisor
Web: http://www.2Checkout.com
Voice: (614) 921-2450
Toll Free: (US & Canada) (877) 294-0273
UK and Northern Ireland: 0871 284 4844
Fax: (614) 921-2451 or (US & Canada) (866) 921-2451