Fixing the error: PayPal module is not properly configured ...

I'm building a paid membership site on Drupal 6 - using Ubercart and Paypal Website Payments Standard as my interface for selling memberships and DVDs.

I already have products that add the subscription & expiring roles using the latest releases of uc_recurring and uc_roles, and uc_paypal... but I would prefer to use the Paypal Subscribe and Buy Now buttons to bypass the cart and streamline the purchase process.

When I select the Paypal subscription feature to add to a product, I get the following message:
"Warning: The Ubercart PayPal module is not properly configured to process payments with PayPal. Please check the Ubercart payment settings."

I have checked all my configuration options (including Paypal Website Payment Standard settings) a couple of times, but am at a loss for what I should change in order to get this to work. I am currently testing against a sandbox seller account. Purchases through the Ubercart checkout process are completing payments and setting up subscriptions properly, so I don't know why uc_paypal_buttons is unhappy with my PayPal config .

PayPal WPS is configured using my sandbox seller email address and "Sandbox" server. PayPal action is Complete Sale. I have a "cancel return URL" (to go with payment complete and payment cancelled URLs in the Paypal subscription product feature settings) ... Shipping address prompt is set to not show and the box for overriding is unchecked (I have no reason to override)... the "sent address" is "billing" and debug logging is turned on.

The message appears on a simple non-shippable product - with or without the Assign Role feature.
(I am assuming I don't need to add a Recurring Payment feature for this to work.)

I assume I will need to define Rules to handle role assignment changes when the various uc_paypal_buttons triggers fire... (do I need to do that, or does adding an Assign Roles feature to the product take care of that for me.)

Thanks for any assistance.

Kathy