You need to create a new billing account and then link it with an existing...

Verify that you are Project Billing Manager for the GCP project. Update the existing project to link it to the existing billing account.

Verify that you are Project Billing Manager for the GCP project. Create a new billing account and link the new billing account to the existing project.

Verify that you are Billing Administrator for the billing account. Create a new project and link the new project to the existing billing account.

Verify that you are Billing Administrator for the billing account. Update the existing project to link it to the existing billing account.