When creating a new custom object, administrators can choose to mark it as deployed or in development. If an object is marked as in development, it is not visible to users or available in tools such as Data Loader, even if they have access to it through their profile or permission set. To make the object available for data import, the administrator needs to mark it as deployed. References:
https://help.salesforce.com/s/articleView?id=sf.customize_deployment_status.htm&type=5