The key prefixes tell you which one is secret
Porego uses two keys, and telling them apart is easy: the public key starts with pk_ and the secret key with sk_. Both come from the integration section of the panel. When they get swapped the error is an authentication failure, and glancing at the prefixes resolves it in seconds.
Setup also includes a payment type. The default is prepaid; if you work with cash on delivery you have to change it and make sure the amount to collect comes from the order. Creating a cash-on-delivery shipment under the prepaid default means the amount is never communicated to the carrier.