Field Types Grayed Out - Why?

Field Types Grayed Out - Why?

Good Morning, I'm working in our Candidates tab, and I'd like to add more fields, but some of the types I'd like to use are grayed out. IE, Number, Date, Number Picklist. Is there a limit to the type of fields you may use? Or is there a way to make these field types available to me? I appreciate your help.