Checklist is marked as readonly.

So you have opened this page because the UI says that the checklist is readonly.

There can be several reasons for it:

1) The custom field is really marked as readonly - by work item permissions or by admin page "Read Only Fields", in such case this is not a problem and system behaves as expected

2)The work item extension to edit checklist is not configured propertly and the extension was not able to locate the editor on the work item form. Please make sure you follow the setup new checklist