Apigee X has a bug which is restricting to add any policy to api proxy. Bug is visible in UI console when trying to click on add policy.
Add little more details 
- what type of subscription - Evaluation/Standard etc you are using ?
https://cloud.google.com/apigee/pricing
2.Provide little more details like snapshots etc to understand the problem statement.
- Reach out to support if possible (if you are in enterprise & above)
its a evaluation portal