Comment by krishicks

These steps didn't work for me; I failed at creating the self-signed certificate:

  yubico-piv-tool -a verify-pin -a selfsign-certificate -s 9a -S "/CN=SSH key/" --valid-days=3650 -i yubikey.pub.pkcs8 -o cert.pem -v
  trying to connect to reader 'Yubico Yubikey 4 OTP+U2F+CCID'.
  Action 'verify-pin' does not need authentication.
  Action 'selfsign-certificate' does not need authentication.
  Now processing for action 'verify-pin'.
  Enter PIN:
  Successfully verified PIN.
  Now processing for action 'selfsign-certificate'.
  Failed sign command with code 6a80.
  Failed signing certificate.

I haven't yet figured out what 6a80 means. Does this guide assume the Yubikey is in any particular state?