I guess Barry is right, you have to define things more profoundly so we could give right answer. Well, To be frank, I have never heard about such error while handling PST file. I guess, the software (Kernel PST Password Remover) which you are using to remove encryption is not able to nail it and therefore showing the error.
Why you don’t give a try to another third party source, whereas you are already paying for something.
Thanks & Regards