Checkout is really three placements — below the line items, a header banner above the checkout form, and a footer below the payment button — sharing one requirement and one styling constraint. The requirement is Shopify Plus, since Shopify restricts checkout UI extensions to that plan regardless of what any app offers; there's no way around this from UpsellPlus's side. The constraint is styling: checkout offers use Shopify's own semantic color tokens rather than arbitrary hex values, so an offer's colors follow the checkout's existing theme rather than being set independently — this keeps checkout offers visually consistent with Shopify's own checkout UI, but it does mean you can't match an exact brand hex code here the way you can on product page or cart.
Beyond Plus itself, a checkout offer needs one more activation step that's easy to miss because it happens outside UpsellPlus entirely: the placement has to be turned on in Shopify's own Checkout Editor. An offer configured correctly in UpsellPlus with checkout as its context still won't appear until that Checkout Editor activation is done — if a checkout offer isn't showing and the offer configuration looks right, this is the first thing to check.