Hello,
** Fabrice Couvreur [2019-04-09 00:58:46 +0200]:

> Hi Vladimir,
> I use AUCTeX and I compile with the command C-c C-c. The configuration you
> proposed for my .emacs file last time works perfectly. So I tried to do the
> same thing with context lmtx and if I add this
> 
> ("ConTeXt" "PATH = /home/viserion/data/context/tex/texmf-linux-64/bin: $
> PATH context --purgeall% s"
>      TeX-run-command nil t: help "Run context (LMTX)") TeX-command-list)
> I get the message
> 
> For information about GNU Emacs and the GNU system, type C-h C-a.
> File mode specification error: (wrong-type-argument symbolp (ConTeXt PATH =
> / home / viserion / data / context / tex / texmf-linux-64 / bin: $ PATH
> context -purgeall% s TeX-run-command nil t : help Run context (LMTX)))

This message tells that something wrong with your settings (I assume the
spaces in above strings are side effect of mail system). The 'cons'
function that you use allows to add only one element to a list (see
Emacs Lisp documentation). I'm not familiar very well with Emacs Lisp
but tested a bit and think this should work:

-------------------------------- 8< ------------------------------------
(eval-after-load "context"
  '(setq TeX-command-list
    (append
      '(
        ("ConTeXt" "PATH=...:$PATH context --purgeall %s"
          TeX-run-command nil t :help "Run context (MarkIV)")
        ("LMTX"    "PATH=...:$PATH context --purgeall %s"
          TeX-run-command t   t :help "Run context (LMTX)")
      ) TeX-command-list
    )
  )
)
-------------------------------- 8< ------------------------------------

I'm not absolutely sure about 'TeX-run-command' (code of AUCTeX uses
'TeX-run-TeX' for TeX stuff). You may test and use 'TeX-run-TeX' instead
of 'TeX-run-command'. Also you may change the 'nil t' to 't
(context-mode)' if you what to edit (expanded) string and only want to
see these entries for buffers set in 'context' mode.

And please check the binary name and it's location (I didn't play with
LMTX yet).

> Fabrice

---
WBR, Vladimir Lomov

-- 
<Flimsy> Anyone here knowledgable in matters of water fowl?  If you
         walk through a park, and a goose starts following you... and
         ends up following you more than half a mile until you reach
         your car... at 11pm.  Is the goose rabid or something?