If everything the user is trying to search comes up Product not found, look up an item in ScratchPad or try to pull a report in Scratchy. There is usually an issue with there Account that needs to be resolved on Amazon's side.
If its a new user the error in the API is normally "Your account is still being setup" Otherwise it may be "{Merchant ID} does not have access to {US Marketplace ID}
If this is the case you would want to have the user reach out to Seller Support's MWS team to see if they can tell why the account is not set up.
In the past we have seen this if a payment did not go through for a professional seller account or if they did not have MWS Developer functions enabled within their Seller Central Settings.