You can write your topic however you want, but you need to answer these questions:
local magnitude = (attachment0.Position - attachment1).Magnitude
Magnitude does give the distance, I think you must be inputting the values wrong. However, I may be wrong on that part.
local magnitude = (attachment0.Position - attachment1.Position).magnitude
to
local magnitude = (attachment0.Position - attachment1.Position).Magnitude
Here’s the full script
local event=game.ReplicatedStorage:WaitForChild("WebSpawn")
local event2=game.ReplicatedStorage:WaitForChild("WebLength")
event.OnServerEvent:Connect(function(player, attachment, pos, canShoot, ropeObj)
if canShoot.Value==true then --if player can shoot
canShoot.Value=false
--setting up the rope
local rope=Instance.new("RopeConstraint")
rope.Thickness = 0.2
rope.Restitution=0.8
rope.Color = BrickColor.new("White")
rope.Visible=true;
rope.Enabled=false
ropeObj.Value=rope
local attachment0 = attachment
local attachment1 = Instance.new("Attachment")
rope.Attachment0 = attachment0
rope.Attachment1 = attachment1
rope.Parent = workspace.Terrain
attachment1.Parent = workspace.Terrain
attachment1.Position = pos
local magnitude = (attachment0.Position - attachment0.Position).Magnitude
if magnitude<300 then
rope.Length=magnitude
wait()
rope.Enabled=true
while rope.Length>1 and rope.Enabled do
wait()
end
end
rope:Destroy();
wait(1)
canShoot.Value=true
end
end)
So I immediately notice you are subtracting attachment0 from attachment0, which would equate to 0. Secondly, if the magnitude is over 300, it will never successfully run the desired code.
Sorry, i made this edit right now and i didn’t noticed it, but i edited it to be attachment1 but still sometimes it gives bigger or smaller distances
Did you alter it from attachment1 to attachment1.Position as I posted previously?
Yes at first i used two variables (start and finish) to store the positions and i realized before posting that they are useless, i edited and that’s when i made the mistakes.
now i have like this, but it still doesn’t seem to work
local magnitude = math.abs((attachment0.Position - attachment1.Position).Magnitude)
local magnitude = (attachment0.WorldPosition - attachment1.WorldPosition).Magnitude
Thank you very much this one works fine! 
the magnitude is never negative, therefore the absolute value function is useless
What if attachment1.Position has a greater value than attachment0.Position?
The magnitude of a vector is the length of that vector. If you think of a vector as an arrow, then the magnitude of the vector is the length of the arrow. Geometrically, it doesn’t make sense for an arrow to have negative length — what would an arrow like that look like? Therefore, the length of an arrow — and likewise the magnitude of a vector — is always positive.
When you subtract two vectors, you get another vector. All vectors have a magnitude and a direction. Regardless of whether the magnitude of attachment1.Position is bigger or smaller than the other attachment, the resulting vector will have its own magnitude which is positive.
Hmm… That makes sense. I wrote this code off the top of my head (but haven’t had the time to test it):
function VectorDistance(a0,a1)
return (a0.WorldPosition.Magnitude - a1.WorldPosition.Magnitude).Magnitude
end