How to find the values that never matched in a full join
What you appear to want is a left outer join on NULL, this will give you only the rows of the left table which do not have a match with the right table:

SELECT    F.Store
FROM      `project.campaigns`  F
LEFT JOIN `project.visitors`   C
ON (
      F.Store = C.Store
  AND F.Date  = C.Date
  )
WHERE C.Store IS NULL