10501 This transaction cannot be processed due to an invalid merchant configuration

Hi,
Is it possible that someone can interpret this error message that came back via email to the storeowner from a user trying to use the Paypal Payments Pro checkout. I've tried it myself, both in sandbox mode and out, and it gives me no error, it just kicks me back to an earlier checkout form. I do have Paypal Express checkout configured and it appears to be working. As specified in the instructions, I have the IPN URL properly inserted, plus I generated an API username/password/key #, all inserted. No other mods to the site

Message received:

Dear store owner,
An error occurred when attempting to initiate the payment-validation
transaction. As a courtesy, only the error "number" was shown to your
customer. The details of the error are shown below.

10501
Invalid Configuration
This transaction cannot be processed due to an invalid merchant
configuration.

Zen Cart message:

Problem occurred while customer 77 Scott was attempting checkout with
PayPal Website Payments Pro.

Transaction Response Details: Array
(
[TIMESTAMP] => 2012-07-13T14:11:55Z
[CORRELATIONID] => 3b14b41782fe
[ACK] => Failure
[VERSION] => 61.0
[BUILD] => 3288089
[L_ERRORCODE0] => 10501
[L_SHORTMESSAGE0] => Invalid Configuration
[L_LONGMESSAGE0] => This transaction cannot be processed due to an
invalid merchant configuration.
[L_SEVERITYCODE0] => Error
[AMT] => 72.91
[CURRENCYCODE] => USD
[CURL_ERRORS] =>
)


Transaction Submission: Array
(
[ACCT] => ************7160
[AMT] => 72.91
[CARDISSUE] =>
[CITY] => Apopka
[COUNTRYCODE] => US
[CREDITCARDTYPE] => Visa
[CUSTOM] => DP-77-1342188714
[CVV2] => ***
[EMAIL] => [withheld for customer privacy]
[EXPDATE] => 032013
[FIRSTNAME] => Scott
[INVNUM] => 77-1342188714-[LongstemOrganizers]
[IPADDRESS] => 97.100.40.162
[ITEMAMT] => 59.99
[LASTNAME] => [withheld for customer privacy]
[L_AMT0] => 59.99
[L_NAME0] => Longstem® Overdoor/Wall Organizer [2]
[L_NUMBER0] => 5253
[L_QTY0] => 1
[NOTIFYURL] => https://longstem.com/cart/ipn_main_handler.php
[PAYMENTACTION] => Sale
[PHONENUM] => [withheld for customer privacy]
[SHIPPINGAMT] => 12.92
[SHIPTOCITY] => Apopka
[SHIPTOCOUNTRYCODE] => US
[SHIPTONAME] => [withheld for customer privacy]
[SHIPTOSTATE] => FL
[SHIPTOSTREET] => [withheld for customer privacy]
[SHIPTOZIP] => 32712
[STATE] => FL
[STREET] => [withheld for customer privacy]
[ZIP] => 32712
[METHOD] => DoDirectPayment
[CURRENCYCODE] => USD
)


Problem occurred while customer #77 -- Scott -- was attempting checkout.

"Invalid merchant configuration" refers to your PayPal account having a configuration problem. Most likely you're trying to use a service for which your account has not yet been enabled. In the case of PayPal's paid subscription services like Website Payments Pro, you must apply for the service online from within your PayPal account, and wait for credit approval.

Regardless, your solution is to contact PayPal Support to assist you. Best to contact them by telephone by using the contact options shown on your browser when viewing your PayPal account (while logged in). Optionally you may log a support ticket manually at www.paypal.com/mts

thank you, will do!