I have this problem as well, I accidentally created a duplicate post about this (now deleted), so here is what I wrote there:
in Configuration > Edit/View > Associations I have defined external viewers for some extensions (in my case editors), e.g. Paint.NET for PNG or Audacity for MP3.
When I press Alt+F3 in TC (=External Viewer) on PNG or MP3, TC shows a dialog saying: File not found! + path to associated program's executable, which does exist, as confirmed using Run dialog in Windows.
In my case the paths are: "C:\Program Files\Paint.NET\PaintDotNet.exe" and "C:\Program Files (x86)\Audacity\audacity.exe".
Alt+F3 on unassociated file types runs default editor, as expected. Please let me know if you need more info from me.
Win10 Pro 64bit, TC 9.0beta2 32bit (didn't work in beta1 either, but did in TC 8.x).
Why? The outer "" are cut by the ini reading functions. Without quotes, the command is seen as:
Command: C:\Program
Parameters: Files\XNVIEW\XNVIEW.EXE
TC8.x was more fault tolerant. I will add it back.
Btw, the "" are added automatically when you define a new type now.