Hi,
Thanks for your response. I actually figured out the issue.
I had the “Development store preview” option turned on. For those who may come across this later, this is found by following: Partner Dashboard → apps → your app → Extensions, where it should be near the top of the page.
Having this on, “Serves work-in-progress extensions on development stores. This overrides the active app version that’s live to users”. Which disallows you from accessing your extensions in the checkout editor it seems.
Thanks!