Quoting David Bremner <david@tethera.net>:

> Jeff Elliott <jeff@fedge.ca> writes:
>
>> [Wed Sep 22 15:49:36 2021]
>> A caller requested output format version 5, but the installed notmuch
>> CLI only supports up to format version 4.  You may need to upgrade your
>> notmuch CLI.
>>
>> When I try to upgrade my notmuch from the Solus package manager, I get
>> that it is already updated.
>
> Yep, this is exactly the problem mentioned in the README on melpa (which
> I'm guessing is where you installed notmuch from). You need to use a
> version of the notmuch CLI that is as new as as your version of
> notmuch-emacs, or an older version of notmuch-emacs. Normally I would
> recommend using the notmuch-emacs package from your distro, but I
> couldn't figure out if it exists in solus.
>
> If you have the appropriate version of the source, then "make elpa" will
> create package you can install in emacs
>
>    M-x package-install-file <return>
>    notmuch-0.31.4.tar <return>
> should work. If you're stuck, I can make such an elpa package for you.

Thanks. I guess I'm still stuck.  I downloaded version 31.4 from:
https://notmuchmail.org/releases/

But when I select "notmuch-0.31.4.tar" for the package-install-file, I  
get an error:
 Wrong type argument: stringp, nil

Would it not make sense to upgrade notmuch to 0.33.1 ?  Which I guess I would
have to do from source, which I haven't done before...