To successfully sign, you must have a valid electronic signature pre-installed on your workstation.
The certificate can be issued by one of the following suppliers: Information Services AD, Borika-Bankservice AD, Infonotary EAD, Eurotrust Technologies AD, SEP Bulgaria AD.
Also you must install  BISS signing software (latest version) on your workstation.
For additional information on installing the signing software, use the instructions posted at the following email address: https://store.borica.bg/bg/b-trust/certificates/biss
Make sure that the current workstation time is set correctly according to the astronomical time and you have selected your correct time zone.