When creating a new instance of WebViewer, the enableRedaction property needs to be set to true for users to create new redactions and the fullAPI or webviewerServerURL properties need to be setup to be able to apply redactions. It's possible to use redactions with only enableRedaction, but users won't be able to apply redactions.
1WebViewer({
2 ... // other options
3 fullAPI: true,
4 // can also use 'webviewerServerURL' instead of 'fullAPI'