If you’re trying to set a part on fire with a script, you need to create a part and put it into workspace. then you need to create a Fire instance and place it inside the part.
local part = Instance.new("Part")
part.Parent = game.Workspace
local fire = Instance.new("Fire")
fire.Parent = part
When you do something like
game.Workspace.Part = true
you’re trying to set the “Part” property of workspace to true. However, the workspace does not have a “Part” property (see Workspace | Documentation - Roblox Creator Hub for the properties of workspace), so you will receive an error. The same logic applies to your second line; assuming there is a part in workspace, Part instances do not have a “New” property.
I did not realize the script was inside the part. Try this:
local part = script.Parent --this is how you reference the part
local fire = Instance.new("Fire") --create the fire
fire.Parent = part --put the fire in the part