This link you are clicking might have expired. This occurs when you open a signature request to view the document on one device, browser or IP address, and do not complete signing the signature request. Then, when you try to reopen the link from another device, browser or IP address, it will deny your access and send a message stating that the link is no longer valid.
If this happens to you, then click on the "request a new link" button (shown below), and then use the same device, browser or IP address to sign the contract.
Long story short: Use the same browser, IP address or device when viewing and signing the contract.