I can´t block the signature

I have a PDF document to be signed by three different persons. The last person must block the signature. However, when he tries to tick the box option that enables to block his signature, it doesn´t appear. I am neither the person who must sign or the person nor the person who has created the document, but I had written on it before it was signed by the first person

How can I enable the blocking option for the last person who must sign?

Thank you in advance