What is Full Disk Access?
Starting with macOS 10.15, Catalina - HTML Email Signature - Mail requires Full Disk Access to add Signatures.
Full Disk Access is a new security feature in macOS that requires some applications to be given full permission to access a user's protected files (in this case, the 'Signatures' folder). This way, only applications that are approved can gain access. In previous versions of macOS, this permission was automatically given to all applications at the time of installation.
- Open the Security & Privacy Preferences pane and select the Privacy tab
- Select Full Disk Access from the list of services
- Click the lock icon in the bottom left corner to unlock the interface
- Enter your macOS administrator password
- Drag and Drop the icon for the 'HTML Email Signature - Mail' into the list