A prg is not included in a project file.
Is it supposed to work at all when exe is launched from command line ?
It does work for exe build/run in IDE.

do &xprg

where xprg full path to prg file

Set path to justpath(xprg)
or
cd justpath(xprg)

don't help.

The error :
File 'xyz.prg' does not exist