Subscription plan selector is empty
You set up your subscription form in test mode, verify it, everything works fine.
You then flip the switch to live mode.
When you look at your subscription form, the plan selector is empty, and the plan details label says: “Plan is undefined”, just like on this screenshot:
You are asking: what’s wrong?
Test mode and live mode are two different worlds in Stripe, with their own set of objects.
The subscription plan you create in test mode is not visible in live mode, and vice versa.
The solution is to create subscription plans also in live mode.
Make sure to use the same plan names as in test mode, otherwise your form won’t work as expected.