Here are some possible causes for this to happen:
1. You have another Stripe gateway enabled along with Subscriptio Stripe. The issue is that both use the similar scripts loaded from the Stripe site, and it may cause JS errors on the page. Try to disable other gateways and test only Subscriptio Stripe.
2. JS error could be caused by the theme, or some other plugin. To check for this, try to deactivate all other plugins, switch your theme to some of the default ones (like Twenty Seventeen) and test this again with only Subscriptio Stripe.
3. If it happens only to certain users - then there's some specific Stripe error hidden under this message. Will need to either contact Stripe support or with some custom code capture the full response received from Stripe.
4. Make sure that your Stripe API keys are correct and entered in the correct fields in Subscriptio Stripe settings - we saw the issue like that a few times.
5. Connection issue - it happens that something is blocked between your site and Stripe server.