Hello Marina,
Thanks for reaching out to us!
I’m sorry to hear that you are having issues with the payment gateway.
Since WooCommerce and payment gateway plugins are not developed by us we cannot give you any support with those.
What I found in the Stripe API Reference about the 400 error is:
400 - Bad Request: The request was unacceptable, often due to missing a required parameter.
For any further help I would suggest to get in touch with the developers of those two plugins.
If you think the issue is caused by The Events Calendar / Pro or Event Tickets / Plus, then first you can run a test with a normal WooCommerce product unrelated to any tickets and events. If you are still getting an error with that test product, then the issue is independent from our plugins.
If all works fine with a WooCommerce product but you are getting issues with Ticket products of our plugins, then there might be a bug in one of our plugins. If that is the case we will try to help you solve the issue.
Cheers,
Andras