This error occurs for a couple of reasons.
1) You do not have a PayPal Pro account. Mijireh requires a PayPal Pro account which PayPal charges $30/month for. Mijireh does not work with PayPal Website Payments Standard.
2) You have not accepted the terms and conditions (merchant agreement) for your PayPal Pro account. You need to log into your PayPal account and make sure you have accepted the merchant agreement before you can accept live transactions.
3) You are not using the correct PayPal credentials. You need to use the PayPal API credentials, not the username and password you use to log into you PayPal account.