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.
