2 Answers
This happens in lot of sandbox environments, just use the project which is already created.
You’re not supposed to create a new project on your own in ACG Sandbox, just use the provided credential & project name to proceed.
ACG programmatically and automatically controls the creation of projects and assignments of IAM permissions on to projects, so that everyone can get a provisioning of his/her Sandbox environment without having to do much. ACG totally holds the control of IAM over the Sandbox account at project level and upwards, so that each Sandbox user can only do things within assigned scope and not touching other which him/her is not supposed to touch.
Sandbox is kinda limited in terms of experimenting some certain GCP concepts. If you want a total control to practice, you should consider registering for yourself a free-trial GCP account, in which Google will give you $300 of credits for use within 3 months.