Why is the document signature signable field is not working?

If you are having this issue, don't stress! Your candidates and clients will still be able to sign. The reason why you can't sign is because you are logged in as admin.

If you are an agency and logged in as such, when you look at an application/profile where there is a document to sign - it won't show the 'submit' button (because you are signed in as admin)

If you sign in in an incognito window, using your URL link and signing in as a candidate or client (best to use a test account) you'll see they are able to sign the document.