The prompt is asking you to set a program as the default email program in W10 Settings/Apps/Default apps. Even if TB is already the default, the scanner software may not communicate with TB if it doesn't support SimpleMAPI, or if TB is 64-bit and the software is 32-bit, and sometimes if Outlook is also installed and interferes with the connection. What is the scanner model?

Thank you for your reply. I am a complete novice so you need to be understanding please! Everything has been working ok but I have obviously changed something. Checked, the default email program in Windows 10 is Thunderbird. The scanner has been working ok sending to Thunderbird. I am using Canon printer as the scanner, model MG7760. The scan will open but will not send to Thunderbird to send as an email. Don

Did TB update recently? Check the update history in the Updates section of Options/General. In the meantime, you can save the scanned document and attach it to a message the usual way.

TB updated today 08/10/2020 - 78.3.2 I need to use PDF so that it cannot be altered. Don

Just realised - I cannot send any file, not just scanned, to TB- prompt says -'There is no email program associated to perform the requested action. Please install an email program or if one is already installed create an association in the Default Programs control panel'

You can try this and see if it restores the connection: download the full installer for the same version currently installed, i.e. 78.3.2, 32 or 64 bit. Run the installer over the existing setup, choose the Advanced setup, set TB as the default email client. At the end of the install, don't run TB. Reboot the computer, then see if the scanner sends files to TB.

The installers for 78.3.2 are here: https://www.thunderbird.net/en-US/thunderbird/all/

Yessssss it works - thank you very much - really appreciate your time. Regards Don