First, ensure that you have set the license key.
Next, check that you have specified the correct private keyring.

If your keyring or a key uses a password, and the key contains subkeys, then the subkeys' passwords must also be set. A common mistake is to copy the OnKeyPassphrase event handler from the sample code and not to add the handler to the OnKeyPassphrase event (using the += operator in C#). Another mistake is that the keyring is not loaded from the disk for some reason (the wrong folder name was specified, etc.). Always check that the keyring does contain the keys.