Can't add category to main menu v4.15.2

I am moving from Shopify back to CS-Cart.
Thanks guys for making this process mostly very easy.
Having trouble adding a category to my main menu.
When I click ‘add’ I am asked to ‘create’ a category
but I am simply trying to ‘add’ an existing category.
Apology for not being able to add screen shots but
the facility is missing.

Please take a look