在APIs/API产品处于PROTOTYPED状态时，是不允许订阅的。需要将APIs/API产品的状态改为PUBLISHED或DEPRECATED，才能进行订阅操作。

以下是示例代码：

# 将API状态从PROTOTYPED修改为PUBLISHED
curl -X POST -H "Content-Type: application/json" -H "Authorization: Bearer accessToken" -d '{"apiState":"PUBLISHED"}' https://api.enterprise.apigee.com/v1/organizations/{org}/environments/{env}/apis/{apiName}/revisions/{revisionNumber}/deployments

# 将API产品状态从PROTOTYPED修改为PUBLISHED
curl -X POST -H "Content-Type: application/json" -H "Authorization: Bearer accessToken" -d '{"approvalType":"auto"}' https://api.enterprise.apigee.com/v1/organizations/{org}/apiproducts/{apiProductName}/approvals