There could be several reasons:
- The list is shared in the read-only mode. In this case, you cannot edit anything.
- The list is public and you don't have writer's permissions.
- You're looking at the print view of the list (see print=true parameter in the URL line).
Remove this parameter to edit the list.
- You wrote some HTML code in the text but forgot to close an opening HTML tag, like <b>.
Checkvist can't handle such cases yet, sorry.