Enumeration and Boolean are the only two options that give you a finite number of possible values. Boolean only: true and false, enumeration only: its list of values. This already suggests that you started the creation of the entity and its attribute at a location where only a boolean or an enumeration is allowed.
Going to the course you mentioned, I see that it works in Mendix Studio, the web variant of the editor. It asks you to add:
a course title
These are all strings and integers or decimals. So if you select any of those and in the properties-pane click ‘Add attribute’, you will get the options you expect. So likely you got yourself in this situation because you selected some page-element the requires a boolean or enum.