Problem with attachments in Thunderbird

I send an email with an attachment. Having attached this file, I see it at the right side of the title. When I double click on it in order to verify that it is the good file, I see the following (in French)

mnt/DATA/JF/Documents/carrelageSDB.pdf n’a pu être ouvert car l’application auxiliaire associée n’existe pas. Changez cette association dans vos préférences.

What have I to do ?

de mon côté
Updated to 21.10 without issue far
Weird changes in Thunderbird though. e.g. PDF attachments opening withing TB; no option to open them with Atril without saving them on the machine.
Or copying an activity to another date; no possibility to edit the second instance. Will dig about this if there is a way to revert some changes.
But no problem with attaching documents to emails, though.

What it says. It can't open the PDF because what you've told it your PDF viewer is doesn't exist.
In TB, Edit -> Preferences -> General, and scroll down and fix your settings.

It is done

But before sending an email, when verifying that the attachment is the good one, I obtain this error message

I have found the remedy. In place of specifying Atril for pdf, set "Always ask". When clicking on the attachment, use of Atril will be suggested, and it works.