if I am unable to set restrictions, how can I make sure that no one but me can edit a document?
Being able to set such restrictions requires a separate permission. And that per Space.
You should ask you space administrator to grant you the ability to set page restrictions.
When you get them. You can click the lock icon at the top op the page to do so.
You can't. That is what restrictions are designed for - locking down access. If you don't have permission to set restrictions, you can't do it.
Talk to your space admins about getting the right to set restrictions in your space.
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