Short answer: the extension is not the problem. Cloneable has nothing it can use yet, either because no export has reached it or because the export that did is now too old.
What is happening
Cloneable's import page does not go and fetch anything from Katapult. It waits for the extension to hand it a model.
If nothing has been handed over, the page has no way to tell the difference between a missing extension and an extension that has not collected anything yet. Both look the same from where it is standing, and so does a third case: an export that did arrive, but was already more than five minutes old, which Cloneable ignores. All three get the same message: use the extension.
So the message is accurate about what to do and silent about why, which is why it reads as wrong when you know the extension is right there.
The fix
Run the export first, then come back.
Go to your Katapult Model Editor and open the model you want.
Open the extension and check it says it is connected.
Choose Export Full Model.
Cloneable opens the import page for you, with your model waiting.
Full detail in Send your Katapult model to Cloneable.
If you have already done that
Three things to check, in this order:
Was the right model open in Katapult? The extension exports whichever model you have open. If the wrong one was open, the export worked and sent the wrong thing.
Did the extension say it was connected? If it said it was reconnecting or asked you to reopen it, the export may not have run. Refresh the Katapult page, reopen the extension, and export again. See Add the Cloneable extension to Chrome.
How long ago did you export? Cloneable only accepts a model that was exported within the last five minutes. A straight-through export lands inside that window, because exporting opens the import page for you right away. If you came back to that page later, or reloaded it after a break, the export counts as out of date, and Cloneable ignores it. You get this same screen with nothing said about why. Export again.
Why opening the import page on its own leaves you stuck
It produces no error, which is the confusing part. It simply sits there with nothing, because there is nothing there it will accept: either no export was ever handed over, or the one that was is now too old to use. Reloading it will not change that.
Going through the extension is the only way to get a model onto this page. If you find yourself on a page asking for a model you thought you had sent, start again from the extension rather than reloading.
If you cannot use the extension at all
Ask us before you try to work around it. Tell us what blocks you, and we will tell you what to do next.