When renumbering views on sheets, Revit requires each of those views to have a unique number. So you quickly drag the views you want on a sheet. You go to renumber a view to #1, and Revit says “nope”. Glancing at the sheet, you see no other view titles that use #1.
What’s the deal?
Almost every view on a sheet has a number, even if the title for the view isn’t reporting it. Elevations, plans, RCPs, sections, 3d views, area plans, and drafting views get numbered as soon as you put them on a sheet. And no two numbers can be the same on that sheet.
Look at the sheet again. Chances are there is a drafting view on there, or an overall plan that doesn’t show the number. Go to the view properties of that view and look for DETAIL NUMBER. One of them is already 1, or whatever number you are trying to use. If you don’t need that view to have a number, change it to something very high (like 99) that won’t get in the way of any other views you add to that sheet. Just because the numbers of views have to be unique, they don’t have to be sequential.