hi i have to do a liner on my 18x36 IG pool and needed to know can i add a light to the pool

You can add a light but it's a lot of work in an existing inground pool, especially if it has a concrete deck. I'm not trying to discourage you just want you to be prepared for it.

If you can add a 1-1/2" wall fitting and min. of 18" of conduit on the backside, you can use a savi melody light. That's probably the easiest way to go because of the small size imo. No niche or bonding required.