Answer:
Signatures can be set as Client Signature or Vendor Signature. In order to sign as a Vendor, you must be an Event Temple user, signed into Event Temple.
You may not be signed into Event Temple in the browser you are attempting to sign. For example, you may have opened your browser with the Document in Firefox, but you're logged into Event Temple in Chrome (this can happen if you're opening a Document link via an email). You need to use Chrome for the Signature box to work correctly.
If you are viewing a Document as a Vendor and it is not allowing you to sign, your Document may be set to Client Signature.
To do update this, follow the steps below:
Double-click or click Edit on the Signature block
Check the Vendor Signature box
Click Apply at the top-right of the edit screen
Additional Information