The revocation certificate could not be created.

This problem was described and resolved here:
https://www.enigmail.net/list_archive/2 ... 09157.html
> Is that the complete text of the error message you have received?
> If it is not, could you please copy/paste it?
> Also, what version of gpg are you running, and have you enabled in
> OpenPGP/Preferences/Advanced "Use gpg-agent for passphrases"?
>
That's it, thank you. Now it works.
However, I don't know: where is OpenPGP's Preferences ?