When you upload a new cohort, once the cohort is validated, the "Create Report" button does not always appear next to the "View Members" button:
But if you refresh the page, the button should appear:
This only seems to happen when you have just created the cohort and the cohort has just validated (So you have clicked the "Refresh" button and have waited until it says X out of N members validated.) When clicking on an existing cohort from the list on the left, the button always appears (in my experience.)