I think you may have created a LIVE mode subscription plan then trying to test it using test mode. For subscription, you need to create new subscription plan for the mode that you want to use it in. Let me know if the following information is helpful:
Subscription plans cannot be interchanged between “live” and “test” modes.
If you have changed between “live” and “test” mode, then make sure to create a new subscription plan in the mode that you are currently using and then use that plan to create a new subscription product.
If you created a subscription plan in “test” mode, then later just changing the mode in the settings won’t be enough. The subscription plan is created in your Stripe account which doesn’t change automatically. So a NEW subscription plan needs to be created in “live” mode and then use that plan in the product.