SetPartCollisionGroup is depracted pleaase use Basepart.Collisiongroup

[m_acch (Mach)]

Player.Character["Left Leg"].CollisionGroupId = 2
What can I change in this line to work with basepart.collision

[icy_oasis]

BasePart.CollisionGroupId is deprecated in favor of BasePart.CollisionGroup.

BasePart.CollisionGroup = "registered_collision_group_name"

Read about the new methods here: Collision Filtering | Documentation - Roblox Creator Hub.

List of the modern PhysicsService members: PhysicsService | Documentation - Roblox Creator Hub.