Mailmerge is an addon, with no support or technical knowledge here. I suggest contacting the author.

Good to know. The add-ons were the only reason why I still used Thunderbird.

If you recently upated Thunderbird and it stopped working after the update.... Are you using the most up to date Mail Merge version for Thunderbird version : Mail Merge 10.4.0 Works with Thunderbird 115.0a1 - *

• https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/

Is the Mail Merge addon actually enabled ? check status in Addons & Themes.

At the link above, there is also a 'Support Email' link to the author of the addon.

It's also worth letting the author know that you are using 'Thunderbird portable' which is modified by portableapps.com

There is a newer version available: 115.9.0 https://portableapps.com/apps/internet/thunderbird_portable

Suggest you try performing an update on Thunderbird Portable first and then see if a fresh install of Mail Merge works ok.