Signatures not merging into document

Issue: When merging signatures into a signed document, the signatures are not merged into the final document.

Possible causes:

1. Incorrect merge field code. Check that the merge field code has no errors. You will want to copy the merge field code directly from the table of merge fields provided in the article, Merge Signatures into a Signed Document.
2. User has a session setting set as ‘High Assurance’. They should set this session to none otherwise they will not be able to merge a signature. See this Salesforce article more for information: Two Factor authenticated user having session setting set as ‘High Assurance’ and Async apex generated PDF attachment, file content shows as blank