This error message appears when the eSignature link has already been opened on another device. The unique link sent to your client can only be used once and is claimed by the first device that opens it.
For example, if you click on the link to test it before your client does, you will claim the link, and your client won't be able to open it.
A less common reason for this error is that the Work Item has been marked as Completed (or has a completed sub-status). If a Work Item is completed, any outstanding Client Tasks or eSignature requests associated with it will no longer be accessible.