Could you, please, be much more verbose? Which product/version, which platform/OS, what kind of signature (digital/electronic)? If digital signature do you have a software (file) Digital ID or a smart card? If the latter, which type, which manufacturer, how it was installed? Do you use standard Acrobat security handler or a 3rd-party security handler? "Default Signing Method" in the "Creation and Appearances Preference" dialog will give you this info. Please, include it in your reply.
A snapshot of the dialog that you get will be useful. It is not clear to me whether the error dialog you get comes from Acrobat/Reader or additional software that you have installed.
Please, give as much info as you can so that we do not go around clarifying what you are doing.