The Signature 365 add-in for Outlook is installed, and configured to add signatures to email, but it is not adding default signatures into email as expected when using Outlook on Windows.
Outlook must be version 2104 build 13929.20296 or later for this feature to work.
To see your current version of Outlook, select the File menu, then in the backstage select the Office Account menu.
The Outlook version is shown in the About Outlook section under Production Information.
Update Outlook to version 2104 build 13929.20296 or later.
See system requirements for more information.