Please use this checklist to see if everything is correct:
1. restgui service handler class has CL_HTTP_EXT_ITS_BASIC
2. SPRO -> Sap screen Personas -> Maintain system has
- 3 letter system id
- "Server.Url" without typo and value in http(s)://<SERVER>:<PORT>/ format
- "Service.Uri" without typo and value as /restgui
3, Note 2026487 is installed
4. If target system is different from source system then you are able to access the following
- http(s)://<TARGETSERVER>:<PORT>/clientaccesspolicy.xml
- http(s)://<TARGETSERVER>:<PORT>/crossdomain.xml
5. role is assigned to user under /n/persos/admin_ui -> User Maintenance -> Search -> Show user -> Role
If you still get error, then if you are using SSO, please check if you have configured login/create_sso2_ticket parameter correctly.
Thanks
Chinthan