The Marketing Allowed Flag should be turned on before it appears in the transaction flow. Here are steps to turn on the flag.

Step 1 - Tap on Setup App

Step 2 - Tap on Features

Step 3 - Turn on the "Customer Marketing Preference"

YOU ARE DONE! Next time you tap  on the [EMAIL] button on your Payemnt screen, you should see the check box under the line where you collect customer's email address.