Hi sampreethi,
May I request you to connect with our support by creating a support case to understand how you are trying to achieve & if there any error you are getting.
For importing test cases it seems you are following the right approach however you can validate by referring to this document here: https://zephyrdocs.atlassian.net/wiki/spaces/ZE6/pages/149749967/Importing+Testcases
For planning and assignment the recommendation is given here: https://zephyrdocs.atlassian.net/wiki/spaces/ZE6/pages/149618862/Test+Planning+Assignments+Structured
I would recommend to log a case for our support to look into if there is any error - https://support.smartbear.com/zephyr-enterprise/message/
I hope this helps.
Regards,
Ramesh
