It is not possible to use an existing Stripe account with WooCommerce Payments.
WooCommerce Payments is built in partnership with Stripe. It uses a special Stripe Express account connected to WooCommerce Payments rather than a Standard Stripe account. The Express account is created when you sign up for WooCommerce Payments and used for all transactions with the WooCommerce Payments service, including payments, refunds, deposits and disputes.
A WooCommerce Payments Stripe Express account can be linked with an existing login for Stripe, but it can not be used with any existing Standard Stripe accounts already connected to that login.
If you wish to use WooCommerce Payments, please complete the sign-up process to be issued an Express account.
If you want to use an existing Stripe account on your WooCommerce store, use the free Stripe extension.