If your job hasn’t been fully delivered and completed to you, have you talked to the seller about it? Are they planning on delivering the full job as promised? I hope so!
You can only leave feedback once an order as been either closed by the buyer within 3 days of an initial delivery. Or if the buyer doesn’t close it, in 3 days, the order is automatically closed, opening up that feedback field for you.
I’ve had cases where a buyer has a large order, and I’ve explained I’ll deliver what I can, and then deliver the remaining work on a date we agree on. Even if the remaining work takes 2 weeks after the order was initially, partially delivered, the order was automatically closed already. Fiverr doesn’t know this, they just assume the order has been done and will close it and allow feedback to be left.