My upsell page isn't showing after someone buys.

systeme.io does not document this failure

Worth saying before anything else. The article covering upsells is a build guide. It sets out what an upsell page needs and never says what happens when one of those things is missing, so nothing below is systeme.io explaining a fault. It is the list of requirements it does publish, which is where to start looking.

The three it states as requirements

• Position. The upsell page must be placed after the main order form, and the downsell page must follow the upsell page.
• Accept and Decline buttons. Inside the page editor you must add them yourself, so the customer can choose to take or reject the offer.
• A resource. After choosing the offer type, you need to add a resource to the page so the customer can access their product after payment.

Where the step is added

Add step in the menu on the left, then Upsell or Downsell as the Type in the Create funnel step popup. Choose a template or build the page from scratch, then Save.

One thing it says cannot be sold there

A calendar event. systeme.io says it is not possible to sell one on an upsell or downsell page. That is a restriction on what the page can carry rather than a requirement for it to appear.

What a digital product hands over on those pages is a course, a course bundle, a tag, or a community. A physical product is selected from the available resources instead.

Price plans are optional in its own telling

systeme.io's wording is that you can also add one or several pricing plans to the payment page. It sits outside the requirements above, so a price plan is worth checking separately rather than assuming it was part of the setup.

What the customer sees on the charges

An accepted upsell or downsell is charged as a separate transaction from the main order, so the customer and the seller both see individual charges for each product instead of one single total.