There is a known reason why this error can occur:
- The key certificate currently selected for use in sign pro PDF is not designated for use with digital signatures. This happens sometimes if the certificate has been purchased from a third-party certificate authority. See suggested workaround and potential solution below.
Solution
Ensure that your key certificate has been designated for use with digital signatures.
To check this proceed as follows:
1. Open Certificate Manager in Windows. You can find it by searching on "Manage" in the Windows search box:
2. Locate the certificate present in Personal -> Certificates
3. Right-click on the certificate and select "Open" from the drop-down menu
4. Click on the Details tab
5. Scroll down the list of properties until you find "Key Usage" and click on it
6. Examine the list of permitted usages to see if it includes Digital Signature as shown below.
If "Digital Signature" is not present there are 2 options:
a. Ask your trust centre provider to add "Digital Signature" to the "Key Usage" list
Temporary Workaround
b. Create a self-signed certificate - it can be generated in sign pro PDF under Settings -> Key Certificates -> Create New.