paypal express checkout error

I'm testing paypal express checkout, but got the following error messgae

We are sorry for the inconvenience. The PayPal account in this store is presently misconfigured to use mixed sandbox and live settings. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002) 10002 Security error - Security header is not valid

I don't know what does this mean?
Can someone help me? Thanks in advance~

It means you've entered invalid information for the API signature, API password, API certificate fields.

thanks for the reply. but, i actually did enter the API information which is retrieved from my PAYPAL Account, and checked twice.
what else probably went wrong?

Is your Zen Cart setting for the PayPal module set to "live" or to "sandbox" ?

The "10002 Security error - Security header is not valid" message means your security credentials are invalid. Make sure your copy/paste from PayPal's site didn't include extra unwanted characters.

DrByte, I set the paypal module to sandbox. and I'm sure I copy the exactly same info as paypal give out.

If you set the module to Sandbox, did you get API credentials from the PayPal sandbox account?

Or are you trying to use "Live" account credentials and the Live server?

If you have Live settings, you must use the Live server.
If you want to use the Sandbox, you must create sandbox accounts and get API credentials from the PayPal sandbox accounts.

Mixing them up will give you the error message you reported.

i think that's the reason...
will go and have a check now
Thanks a lot

I get the same error.... how do I go about getting the original settings for the live version of Paypal Express? I have been looking at the paypal pages and have not found where to get the API username and API password etc... Can someone point me in a direction or give me a hand? I did it once but can't seem to find the info on the Paypal site again.

Thank you so much!
Site is down until I can get this fixed (it's a new site).

On the outside chance that someone else is as dense (and blind) as me, the information can be found in the "profile" section on the main log-in page.