Hi Sonia J S
From the image you've uploaded, it looks like you're using Azure DevOps Test Plans, but currently have basic access, which might limit your ability to create or manage test cases.
Navigate to Organization Settings -> Billing -> Click on Start Free trial for Basic+Test Plans
Then Install the extension Test & Feedback which you can see on the Test Plan page, Click on Install and then Go to test plans and create a new test plan.
Alternatively, Go to Boards -> Create new Item -> Click on 3 dots -> Click on Add Test. It will be reflected in Test Plans.
Additional References:
Hope this helps!
Please Let me know if you have any queries.
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