As a PO, how do you prioritize when your client says "I want all at once !"

If they are the voice of the customer, or fulfilling the role of the PO in Scrum, it is incumbent upon them to be able to prioritize the work to be offered.

Part of it is a recognition of what is actually achievable based on capacity and capability of those doing the work.   Insisting that everything is urgent without acknowledging what can realistically be achieved is grossly unfair and borderline incompetent.

What is the business currently using to help them determine what can be completed within a given time frame?   Keep in mind the 8th Agile Principle:  "Agile processes promote sustainable development.   The sponsors, developers, and users should be able to maintain a constant pace indefinitely."   Agile/Scrum is definitely not about cutting corners or squeezing IT staff in order to get stuff done.

One tip I can give you is to ask them, of all the items they want, which one would they care about the most if there was a production defect with it?   Which one would they care about the least?   That information is usually telling as far as what is most/least critical.

As the Product Owner, you are accountable for the value which is delivered Sprint by Sprint. The ordering of the Product Backlog is ultimately your call.

If I was you, I'd invite client representatives to Sprint Reviews so they can see what is available each Sprint. Whether they attend is of course up to them, but you should provide them with the appropriate opportunities for engagement.

If the customer needs all at once or in 6 months , then what is the need to do agile? Waterfall works best where you get presumably all at once.

Who decided to use Agile when the client has the mindset to get everything at one go.

Extending to what Ian has mentioned...

This is the situation where client lives in waterfall world and IT organization is in Agile mode.

My 2 cents: PO needs to sit closely with customer and understand the requirement as a whole. Then he/she needs to slice the requirements vertically (and of course prioritize them) so that at any given point of time a working functional module is ready to be demoed or delivered. Involve customers in the reviews. Entice them by giving them opportunity for early peek and thus early feedback. I am sure customers would love to get early peek into the system provided we give them a plan.

In addition to asking "what do you want first?" or "what is more important to you?", I have used another question that works well, which is "what will provide you with the most benefit/value?". Perhaps that will help rank the PBI's.

It sounds like the customer may be missing one of the major reasons for using Scrum.

Try forgetting about priority (just for one moment), and focus on what you could do with the Increment at the end of a Sprint.

Would you expect the customer to see this Increment? What might happen if the customer is interested in what you've done already? How might they react? What would you do with what you learn from their reaction?

Based on this, what would they want to see first, and what do you think they need to see first?

That should help you set the priority.

Don't be afraid to explain to the customer how their involvement helps you to deliver a better product.

If the client just want to deliver the whole thing at once, then I think Agile is not applicable. Based on what you have said, it looks like the client already has the complete picture of the system they want. Meaning, they are already decided with the features, and most likely they're not going to change their mind.

But if you really want to use Scrum, my suggestion is to arrange the Product Backlog based on the features' dependencies. Features less dependent to other features should be the highest priority, then features with the most dependencies the lowest. Just to make the sprints execute smoother and to avoid blocking during development.