Did you realize your Custom Flow is set as the wrong type of flow? Here's how you can switch between types of flows:
If you are unable to select subscription flows, see our video on how to request permission from Facebook here.
Go to your Custom Flows section under Flows in the side menu. Select the Flow you'd like to change the type of:
Select the three dots located to the left of 'Save And Exit'. Open Flow Settings:
Select the type of flow you'd like to change it to:
For more information on custom flows you can learn more here.