SelloSello support
First, install your certificate and its private key in macOS Keychain. They are usually supplied in a .p12 or .pfx file.
Your certificate is missing
Open Keychain Access and import your certificate. Check that it includes a private key and has not expired. Quit and reopen Sello to refresh the list.
The PDF cannot be signed
Try an unencrypted PDF. Documents with altered signatures or permissions preventing additional signatures are rejected to protect their integrity.
Start from Finder
Right-click a PDF and choose Open With → Sello.
Contact support
Email contact@negativeepsilon.com with your Sello version, macOS version and the error message. Do not send private keys, .p12/.pfx files or confidential documents. If an example is needed, use an anonymised copy.