As mentioned in this post, the datasetId is the name of the cloud project.
datasetId
It is working fine, if my project has a name like \"project-name
project-name
The Cloud Datastore API is not currently supported for projects whose name begins with a domain/colon (e.g. "example.com:project"), but we plan to support this in the next version of the API.
https://github.com/GoogleCloudPlatform/google-cloud-datastore/issues/82