It's probably a bad syntax/entry somewhere along your code, could be an issue with the path or arguments, I'd test the service manually a bunch before putting on NSSM, make sure you have the correct flags/options
You can also set Standard Output through NSSM and that would give you a clue of what's going on.
If there's anything that'd interrupt the dependencies for that service to run, you'd also get that error.
This is how I install a service using NSSM with Powershell (make sure nssm is added to the PATH):
nssm.exe install "SERVICE NAME" "PATH TO FILE"
nssm.exe set "SERVICE NAME" AppParameters "<CHECK YOUR FILE OPTIONS>"
nssm.exe set "SERVICE NAME" AppStderr "PATH TO SAVE OUTPUT"
nssm.exe start "SERVICE NAME"
If you or anyone happens to see this post and need assistance, I can probably help you, just post more details on what needs to be done and what you're EXACTLY trying to do.