Why isn't a discipline model existing in Model Components shown in Requirements?

You will not be able to see the content you have created in Requirements section unless these are not added to the requirements table using Requirements Setup. This section is aimed to compile models, objects, groups, geometries and properties together to create the requirements table. Easily move the items by holding the left-click from left (Model Components) to the right (Requirements). The next step would be to set requirements in your project guideline.