I do not see how using an elevated root process can prevent your child process from running… but then that is a Linux issue and not something we can really do much about here in a Blynk forum. Try asking around on various Linux forums.
All I can suggest otherwise is to try breaking down your sketch, or using a smaller test sketch, just controlling the light, and compare with all your available variants… sudo, not sudo, Blynk, not Blynk, alternate ways of controlling the light (e.g. direct JS code instead of what appears to be a Python sub-process), double checking path and user access, various combos of each, etc and so on.