Sounds like you clicked on Gray Inactive Worksets.

Click on Worksets in the Ribbon. Look at the check boxes at the bottom: User Created, Project Standards, Families and Views.
I've noticed that a lot of Revit users don't realize that every View and Family has its own "system workset". So, making a new User Workset for "Sheets and Views" is not the correct approach. Edit: As Dave mentioned, also check "Gray Inactive Worksets" in Manage Collaboration Tab.

Let's back up a step.
What is it you're trying to accomplish by assigning all Views to a (presumably locked) Workset?
As Cliff says, every View has it's own internally assigned Workset.
But if you lock out all Views, no one would be able to create any new Views or adjust anything

We can't create a workset and assign sheets and views to it. That isn't possible and not how Revit works. As Cliff commented, views and sheets are a workset themselves automatically and that is managed internally by Revit. The same is true of families and project standards.

I use a library as a metaphor for worksets. We create shelves (worksets) for model elements (books) and assign them to the shelves. Revit creates shelves for view and sheets and the books (annotation and viewports on sheets) are assigned to the shelves automatically when we add them to views or sheets.

Fundamentally Revit is the librarian, keeping track of the books and who has signed them out, and the librarian's purpose is to prevent any book from being borrowed by more than one person at a time. In Revit that means no element can be edited until the current borrower has relinquished control of it.

Revit allows us to borrow elements simply by interacting with them, such as move, rotate or changing a property. As long as nobody else has already done so Revit just gives us the element, we become the borrower.

If you don't want other people to edit certain sheets or alter certain elements without first bumping into you, then you can make View worksets Editable. That makes you the Owner of a workset, everything on that shelf is yours. Now any person that tries to edit something belonging to that shelf will get a message that you are the borrower/owner and they'll have to ask you for permission (Editing Request) or wait for you to Synchronize to relinquish it/them.

The common practice of creating working modelling views came out of the desire to preserve some views for using on sheets and others (working) for more general modelling activities. This way a team knows which views are on sheets so they must be altered much more carefully and only by certain people perhaps. Working views are less rigid since they aren't meant to go on sheets.

Is there a way to lock the views on sheets so they can't be altered, i.e. adjusting the view template, etc? I'm pretty sure the answer is no, that you should make sure your team knows not to make those adjustments to those views, but it never hurts to ask.

Short answer, no. Training and setting expectations (standards) goes a long way.

You can Pin the Viewport on the Sheet. That makes it a tiny bit harder to move them accidentally.

You can select the views/sheets the workset dialog and click Editable or you can select a sheet view the in the Properties Palette and right-click Make Workset Editable or Make Workset Editable With All Views. As long as you remain the owner/borrower of these sheets and views other people will find whatever they try to do with/to them will get the warning message that you/someone else own them and they have to create an editing request and wait for you to relinquish them.