If Celery isn't available then what other vegetable or green vegetable can I use as a substitute for the making of Harira Soup?
Celery has a very specific flavour profile.
The closest substitute for a soup would be parsley leaves and stalks.
To get even closer to the flavour, you could add celery salt or vegetable stock powder/cubes. In my experience, the most dominant flavour by far in soup stock is the celery.
I would actually try fennel. It will give you a lot more of the anise flavor, so perhaps don't put so much in it. A bit of fennel and parsley as suggested by Carmi might be a good combination, at least in terms of flavor.