Unless the pergola is listed on the plan as common property, it looks like it’s down to you.
If you’d bought the unit 18 months ago, you could have claimed it as a non-structural defect. If the thing was falling down, you might (maybe) have claimed it as a building defect.
But as I say, unless it’s clearly defined on your strata plan as part of common property – and I’m assuming it’s a separate structure in your garden – then it’s down to you.
Prepare for more fun and games when you tell them what colour you plan to paint it.