It is not possible to promote proxy configuration to staging environment because endpoint for promoting requires name of environment from which it is promoted, see

Proxy Config Promote
/admin/api/services/{service_id}/proxy/configs/{environment}/{version}/promote.json POST 

There is workaround: update proxy configuration by

/admin/api/services/{service_id}/proxy.xml  PATCH

and probably this will be fixed in this PR:
https://github.com/3scale/porta/pull/1326#discussion_r336540612

Dev Notes

• Deprecate the behaviour in 2.7: on proxy update, a proxy config is automatically created. It will not be the case anymore in 2.8. customer should use the promote endpoint introduced in 2.7
• remove the behaviour in 2.8