Yes, I double for configuration and point the GAM. Actually, the code was working almost 1 year, but recently just stopped working. I have tried several options including reinstall GAM, but the same error. I can run command, working fine, but not working with PHP exec function as I mentioned on the post.
I'm still struggling to figure out now, it's urgent too as my institution is now registering new staff and students, we need to generate email for them automatically with that PHP function, but now it's not working!