Can not sign a PDF more than one time

I have inserted two SignatureFields and when I try to sign the pdf with the second signature i got this error message:
class com.aspose.pdf.exceptions.PdfException: You cannot change this document because it is certified.
Im using the demoversion 21.10 of Aspose PDF. Are there some limitations inn the demo?
I can not find any examples on how to solve this.

@ron.inputdata.no

Could you please share your sample PDF document(s) along with complete sample code snippet so that we can test the scenario in our environment and address it accordingly?