The most common reason for this is that someone's already paying for the subscription.
You'll see at the end of the text below that it says 'This setting cannot be changed once there is at least one active sign-up.'
So, what's my recourse?
One path is to duplicate the subscription and keep everything the same except for that renewal setting -- flip that on. Then, you can transfer the individuals from the old subscription to the new one.
How do I do this?
Click Memberships > Subscriptions and Packs > Navigate to the subscription, click its dropdown menu and duplicate it.
Scroll to Auto-renewal settings and flip this toggle on.
After saving the new subscription, navigate to the old one > Click on it and scroll to its customer list at the bottom > Click on the first customer.
To the right of customer's subscription you'll find a dropdown menu > Click it and select Transfer.
Select the new membership you just created > the choose your preference for timing of transfer and charging options.
Repeat this process into all customers have been transferred to the new one.