If you set the system preference "BiblioAddsAuthorities" in the Authorities tab to "allow" you will be able to edit the 100 fields. Since this is a common point of confusion for new Koha users, I suggest it might be a good idea to change the default.
I'm doing some research on authority bugs and ran across this old one. There isn't a syspref called "BiblioAddsAuthorities" anymore. Based on the description in this bug, I'm guessing this syspref was rendered unnecessary by AutoCreateAuthorities and RequireChoosingExistingAuthority. Marking as "In Discussion," but I'm wondering if this bug could actually be marked "Resolved"? What does the community do with old bugs that are obsolete or no longer relevant?
I found: Bug 30025 - Split BiblioAddsAuthorities into 2 preferences This patch splits the system preference BiblioAddsAuthorities into two new system preferences that more clearly define what the settings do: 1- RequireChoosingExistingAuthority: this preference indicates whether a cataloger must choose from existing authorities, or if they can enter free text into controlled fields 2- AutoLinkBiblios: this preference determines whether Koha will attempt to link a new record to existing authorities upon saving. In conjunction with the existing preference, AutoCreateAuthorities, unmatched headings will either be linked to a new authority, or remain uncontrolled And it looks like RequireChoosingExistingAuthority is already set to "don't require" by default. I am closing - please re-open if there is still something that needs to be done here!