Look at where the shortcut you use start GIMP points.
I have never heard of flatpaks before, but a quick googling indicates the command should be something like
flatpak run org.gimp.GIMP
Thank for your continued help. I usually start any flatpak application from the Cinnamon menu, i.e. as a GUI application rather than using the command line (simply because, at my advanced age, I have time left only for using my PC to do work, not for trying to become an expert in how it works). The start menu icon for GIMP gives no information (that I can find) that shows what the invocation string is. However, searching among the flatpak help system I was able to confirm your suggestion about the required command string. And indeed this works when entered from the command line. But entering this string ("flatpak run .org.gimp.GIMP") in Scribus preferences takes me directly back to where I started this forum Post: error message "The program flatpak run org.gimp.GIMP is missing or failed to open".
Well, which is it ? If it is missing why not tell the user from which location it is missing? If it failed to open why not tell the user the reason code, given by the kernel, for this failure to open? The location question is made a great deal more complex by trying to definitively locate the GIMP executable. So far I have tried the following locations within my 'out of the box' Cinnamon install:
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/stable/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/bin/gimp-2.10
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/stable/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/bin/gimp
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/stable/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/lib/ about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/stable/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/share/ about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/stable/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/export/bin/org,gimp.GIMP
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/export/share/applications/GNU Image Manipulation Program
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/active/export/bin/org.gimp.GIMP
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/active/export/share/applications/GNU Image Manipulation Program
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/active/files/bin/gimp-2.10
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/active/files/bin/gimp
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/active/files/lib about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/stable/active/files/share about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/current/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/bin/gimp-2.10
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/current/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/bin/gimp
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/current/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/lib/ about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/current/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/files/share/ about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/X86-64/current/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/export/bin/org,gimp.GIMP
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/303711bf7f4af1bb74fdd6425db6899663c94a833d9d40511fc4c1d470ac04c0/export/share/applications/GNU Image Manipulation Program
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/active/export/bin/org.gimp.GIMP
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/active/export/share/applications/GNU Image Manipulation Program
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/active/files/bin/gimp-2.10
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/active/files/bin/gimp
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/active/files/lib about 5 possible paths from here
/var/lib/flatpak/app/org.gimp.GIMP/x86_64/current/active/files/share about 5 possible paths from here
Then there is the tree like structure starting from /usr/lib and /usr/share. I've run out of time to test these.
None of those locations tested to so far are valid, and this has taken many hours of work, leading me to conclude that it is not possible to invoke Gimp 2.10.28 (flatpak - the only available version now) from Scribus 2.5.7 . This seems like a defect - which is surprising to me given that both of these applications have existed for many years (admittedly, not in Flatpak form). The quicker, but much less efficient, way to solve this problem is to temporarily stop work in Scribus, start GIMP and do the editing there, save the edited image file, then reload it into Scribus - then repeat as often as necessary to get the image adjusted as needed. Not very user friendly, is it?