Please post a screenshot of the error. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

thanks for considering my problem.

You've set up end-to-end encryption for your account and told Thunderbird to 'Store draft messages in encrypted format'. For whatever reason the key or cert you specified doesn't exist. So you'd need to fix your settings.

At the top right of the Thunderbird window, click the menu button ≡ > Account Settings - End-To-End Encryption

Many thanks - looks like draft saved OK now. Not sure how that happened to get end to end encryption . . . yes I know it doesnt happen on its own . . . .

But I still dont know what a "keyring" is, or where to find it - despite doing much searching on this site. It would be helpful if popup messages and documentation were better.

And yes, I have contributed to Thunderbird.

Helpful - yes of course, thanking you, appreciated.

But I still dont know what a "keyring" is, or where to find it

It's where Thunderbird stores your OpenPGP keys. At the top right of the Thunderbird window, click the menu button ≡ > Tools > OpenPGP Key Manager

There is plenty of documentation about OpenPGP. The FAQ may be a good start. https://support.mozilla.org/en-US/kb/openpgp-thunderbird-howto-and-faq

Many thanks for the info as to how to get to the "OpenPGP Key Manager" and for the link.

My point is that the message said go to the "keyring", it didnt say go to the "OpenPGP Key Manager".

For the NEW to Thunderbird a lack of clarity wasnt helpful - if it had been clear I might not have had to request help - the help which I much appreciate - but the time could have been used to helped someone with a more serious problem.

Many thanks for your help and pointing me in the right direction.