In order to publish an API in wso2am-3.X.X, you have to provide the "business plan" and "endpoints".
If any of the above requirements are not satisfied, it is indicated on the lifecycle/overview page, and you need to navigate to relevant sections and provide the missing information such as endpoint URL and business plans.
Fig 1: Lifecycle page with conditions to publish the API
Fig 2: Overview page with conditions to publish the API
In your situation, you have provided the endpoint but have not selected the business plan. Please select the business plan, save it and try to publish the API
For more details please refer https://apim.docs.wso2.com/en/latest/learn/design-api/publish-api/publish-an-api/