When adding a dynamic document is throwing error "Required field is missing"

[Lani22520346n6c7]

My team and I are not able to find out where the error "Required field is missing" comes from. What we have set up is a button to send a static document, but need to also send dynamic documents. When the runtime variable is added to the attachment tab, the above error is thrown. 

Does anybody have an idea of what could be the cause? How do we fix this?

[Lani22520346n6c7]

More information: Error displays on click and on the line execution to invoke AdobeSign Agreement Template.