Getting an error in Paypal Express

I'm trying to get my store site setup on my test store first and I keep getting the following error when trying to test Paypal Express:

We are sorry for the inconvenience. The PayPal account authentication settings are not yet set up, or the API security information is incorrect. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002)

The settings for Paypal Express are:

Express Checkout Shortcut Button
Off

Set Order Status
Processing [2]

Set Unpaid Order Status
Awaiting Payment [8]

Set Refund Order Status
Refunded [9]

Express Checkout: Require Confirmed Address
No

Express Checkout: Select Cheapest Shipping Automatically
No

Express Checkout: Skip Payment Page
No

Express Checkout: Automatic Account Creation
No

Payment Action
Final Sale

Transaction Currency
Only USD

PayPal Page Style
Primary

Sandbox - while testing

I have verified the API credentials are correct and the URL is set correctly. What did I do wrong?

Okay, figured it out - it doesn't work in sandbox mode.

Right. You can't use Sandbox Credentials in Live mode. And you can't use Live credentials in Sandbox mode. And Sandbox testing can only be done from your computer; you can't have multiple people testing concurrently in the sandbox.

I am getting the same exact error - new store. However, not in sandbox mode.

Settings are:
o you want to enable this payment module?

True

Sort order of display.
Sort order of display. Lowest is displayed first.
10

Payment Zone
If a zone is selected, only enable this payment method for that zone.
none

Express Checkout Shortcut Button
The Express Checkout Shortcut button shows up on your shopping cart page to invite your customers to pay using PayPal without having to give all their address details on your site first before selecting shipping options.
It has been shown to increase sales and conversions when enabled.
Default: On

On

Set Order Status
Set the status of orders paid with this payment module to this value.
Recommended: Processing[2]

Set Unpaid Order Status
Set the status of unpaid orders made with this payment module to this value.
Recommended: Pending[1]

Set Refund Order Status
Set the status of refunded orders to this value.
Recommended: Pending[1]

Express Checkout: Require Confirmed Address
Do you want to require that your (not-logged-in) customers use a confirmed address when choosing their shipping address in PayPal?
(this is ignored for logged-in customers)

Yes

Express Checkout: Select Cheapest Shipping Automatically
When customer returns from PayPal, do we want to automatically select the Cheapest shipping method and skip the shipping page? (making it more express)
Note: enabling this means the customer does not have easy access to select an alternate shipping method (without going back to the Checkout-Step-1 page)

No

Express Checkout: Skip Payment Page
If the customer is checking out with Express Checkout, do you want to skip the checkout payment page, making things more express?
(NOTE: The Payment Page will auto-display regardless of this setting if you have Coupons or Gift Certificates enabled in your store.)

Yes

Express Checkout: Automatic Account Creation
If a visitor is not an existing customer, a Zen Cart account is created for them. Would you like make it a permanent account and send them an email containing their login information?
NOTE: Permanent accounts are auto-created if the customer purchases downloads or gift certificates, regardless of this setting.

Yes

Payment Action
How do you want to obtain payment?
Default: Final Sale

Final Sale

Transaction Currency
Which currency should the order be sent to PayPal as?
NOTE: if an unsupported currency is sent to PayPal, it will be auto-converted to USD (or GBP if using UK account)
Default: Selected Currency

Only USD

Store (Brand) Name at PayPal
The name of your store as it should appear on the PayPal login page. If blank, your store name will be used.

Allow eCheck?
Do you want to allow non-instant payments like eCheck/EFT/ELV?

Instant Only

PayPal Page Style
The page-layout style you want customers to see when they visit the PayPal site. You can configure your Custom Page Styles in your PayPal Profile settings. This value is case-sensitive.

API Signature -- Username
The API Username from your PayPal API Signature settings under API Access. This value typically looks like an email address and is case-sensitive.
Primary

API Signature -- Password
The API Password from your PayPal API Signature settings under API Access. This value is a 16-character code and is case-sensitive.

API Signature -- Signature Code
The API Signature from your PayPal API Signature settings under API Access. This value is a 56-character code, and is case-sensitive.

PayPal Mode
Which PayPal API system should be used for processing?
Choices:
For choice #1, you need to supply API Settings above.
1. PayPal = Express Checkout with a regular PayPal account
or
for choices 2 & 3 you need to supply PAYFLOW settings, below (and have a Payflow account)
2. Payflow-UK = Website Payments Pro UK Payflow Edition
3. Payflow-US = Payflow Pro Gateway only

PayPal

Live or Sandbox
**Live: ** Used to process Live transactions
**Sandbox: **For developers and testing

live

Debug Mode
Would you like to enable debug mode? A complete detailed log of failed transactions will be emailed to the store owner.

Off

You need to fill in your API credentials: username, password, signature.
https://www.zen-cart.com/getpaypal

I did. I just left them off of here for security reasons.
I used the link to paypal, copied all three values as paypal gave them to me.

Then contact PayPal to get their help in figuring out what's wrong with your account. https://www.paypal.com/mts

Having your PayPal Debug Logs on hand will help them help you: http://www.zen-cart.com/showthread.php?61199-Debug-Logging-PayPal