Can't sign received docusigned document

I received (via email) a PDF that had been docusigned by a customer. I need to also sign the document, but the Sign option is grayed out.

The signature in the document is detected as valid.

In document properties, security is set to "No Security" and it says Signing is "allowed". How can I Fill & Sign?

I tried with another document that had not been docusigned and it worked.

Thanks,

Tim