Head to the sets main page and you are looking for an orange heading that says "See any inaccuracies" (or close to that).
Below the heading is a blue link you can click on, follow the prompts and provide details as to the missing cards, including the checklist.
On a mobile device the heading/link appears to be near the bottom of the page. Last night on a PC it was on the right of the page.
Hopefully this is helpful. Unfortunately,.some members seem to feel putting people on blast is more productive than providing information.