That error is for the customer side (not the merchant). Looks like you have done many tests with ONE customer (most likely using the same email). Then you didn’t clean those up by cancelling the test subscriptions. Stripe has a maximum of how many subscriptions ONE customer can subscribe too.
Log into your Stripe merchant account. Then go to the “Customers” and the “Subscriptions” interface and search with that email address. Clean the existing subscription entries that you have with that email address (that you are testing with). Make sure to clean it up from both the “test mode” and “live mode” of your Stripe account.
Also, Stripe has a minimum amount limit. So you will need to set the amount to something like $1.