This thread from @ ArkaNoid_76 is closed to new posts: Unable add devices over CLI
FYI, I had exactly the same problem, and I have the solution.
The problem was that I was in directory /root
when issuing the command:
sudo -u librenms lnms device:add ...
...
In Process.php line 350:
Unable to launch a new process.
I found the problem using strace:
[pid 51159] chdir("/root") = -1 EACCES (Permission denied)
The solution was to cd /opt/librenms
before issuing the command.