As per the error message, you cannot delete a document where it would cause a signers to have 0 documents visible them. Before deleting the document in question, you will need to either reset its visibility (i.e. make them visible to everyone in the transaction) or delete the signer(s) that can view the document.
Step 1 : Changed Package status to DRAFT
Step 2 : Removed the Signers on the package
Step 3: Changed the package status to Send/InProgress
Step 4 : Changed the package status to Draft
Step 5 : Removed the documents
Step 6 : Changed the package status to Send/InProgress