If I remember correctly, you have a young dog that you are trying to train in both service work and personal protection. I also remember that the service work that you would be having the dog do would be picking things up and holding doors. With your physical limitations and just starting with all of this training on a 12-week-old pup, you NEED to be working with an instructor for at least the protection part of his duties. But I don’t think it is a good idea to hope to get both out of one dog for many reasons.
What you are doing (as far as trying to get him to bark) is about what I would do, except that you need to focus on prey drive building. Based on a couple of your posts that I have read, he has no desire to play with a toy or tug. Please, if at all possible get the video Drive Focus & Grip. It will show you how to do this. But in the mean time, you need to try something else, as the food is not working.
Go back to the back tie, and instead of holding a treat in front of him and giving a command that means nothing to him yet, tease him with a rag on a rope. Just an everyday, cheap white rag. Tie a rope (and attached to a pole for even more range and control) around it and kinda flip it around just out of his reach. Swing it and drag it back and forth. Jerk it so it moves just a little, and all just out of his reach.
I would not be giving a command yet (picture telling the dog to sit, sit, sit, sit until he does, but still has no clue what you are asking) but would wait until he is barking, and when he does, tell him “Good _____” (We use "Reveir!" for barking.It is used later in the bark and hold.)When he finally does bark, give him the satisfaction of catching the rag. He is probably teething a little, so no real tugging, but a very little resistance with out causing pain should be OK (or just wait until he is done teething for even better training). Keep holding the rope so that when he drops the rag you can go right back into the teasing.
Please stop BEFORE he gets bored and starts looking around for something else to do. If this happens, put him back in his crate for a while and try again later.
If you are working with a trainer, they can have a second dog doing bite work while your pup is tied up and watching. Sometimes seeing and hearing another dog working and barking gets them worked up.
Keep in mind that this training is a VERY physical kind. There will come a time when this dog, if he gets his drives up, will be pulling and lunging at the end of a leash, and even just building drive can leave me out of breath. You will be doing a lot of back tying and will likely need a second handler if your physical limitations are severe enough to need a service dog. And you WILL need to work with a trainer and helper (for SO many reasons, among them, this is not a type of training that even a intermediate trainer can do alone.). It is a very exciting discipline, and there are so many rewards when that bond between you and your dog develops with the understanding that you will share. I wish you luck!!!
Jessica