When defining a command as a constant within a Makefile:
MY_COMMANDLINE="prog arg1 arg2" and using this constant later to get the command invoked, you could get:
/bin/sh: prog arg1 arg2: not found although prog definitely exists.
Explicitly giving the whole path of prog does not help.
There are other questions regarding shell saying "not found" (e.g. this or that). By linking this question others may find this possible cause easier.