Maybe mark this solved.. Thanks everyone for the help.

Can this [i386] installation be easily upgraded to 64-bit with some gimmick?

Intresting problem. Google found
https://wiki.debian.org/CrossGrading

One of the steps is adding amd64 architecture, which should allow installing the wanted  (64-bit-)program.

# dpkg --add-architecture amd64

And no, there is no easy way to convert to 64bit. You should download an amd64 medium and repeat the installation.

Shite... So maybe darktable only available as 64-bit?? And maybe I have to install 64-bit devuan..? Can this installation be easily upgraded to 64-bit with some gimmick?

$ lscpu
Architecture:            i686
  CPU op-mode(s):        32-bit, 64-bit                                                 ### comment added, This line?
  Address sizes:         36 bits physical, 48 bits virtual
  Byte Order:            Little Endian

# deb cdrom:[Devuan GNU/Linux 5.0.1 daedalus i386 - desktop 20230914]/ daedalus non-free-firmware non-free main contrib

deb http://deb.devuan.org/merged/ daedalus main

deb http://deb.devuan.org/merged/ daedalus-updates main

deb http://deb.devuan.org/merged/ daedalus-security main

deb http://deb.devuan.org/merged/ daedalus-proposed-updates main

So was thinking if it is not available to m4500 precision hw architecture platform or is missing in some repo trace??

Rawtherapee installation shows it is 32-bit installation..

Tried also apt update and apt policy darktable from terminal.. Does not find it from terminal or thru synaptic.

Checked also lscpu if system/installation is 64-bit and it should be cabaple running 64-bit software..

How about if it is not available to intel i7 hw architecture from repo??? Says something about amd64 on those screenshots?

How to add non free if it contains non free parts, but shouln't non free soft be included in deb.devuan.org/merged daedalus main ??

Alternative on my Daedalus:

# apt list darktable
Auflistung… Fertig
darktable/stable 4.2.1-4 amd64

Anyhow, have a look at the right top of this web page. There are 5 links. Use Devuan.org and scan through the available information, it might be helpful for a beginner.

You may want to add the additional options like adding "contrib non-free non-free-firmware" to each line.

But at any rate it should still show up as it is there with "main".   

Have you run "apt update" before looking for it? Or, are you using synaptic?  A better question is, how are you trying to look for it and install it, with a terminal using "apt" or with the gui program synaptic?  If using apt you must might first want to run "apt update", and if using synaptic, click on "Reload" first, before searching.

Hope this helps.

rawtherapee is there and gimp, but can't find darktable.

What does your /etc/apt/sources.list file look like?