This issue came up today, and I wanted to document the solution for future reference. A user was working on a model, in Revit 2018, and was trying to create a new Sheet. On the View ribbon tab, on the Sheet Composition panel, the Sheet tool was inactive.
She was working on a project that had Design Options, and one of the Design Options was set active. The solution was to use the Active Design Option selector on the Status Bar and set it to Main Model. You can only create new Sheets in the Main Model.