macOS Sonoma Mail Won't Allow Adding Existing Signature to email Account

I have added a new email account to my macOS Mail app. When I attempt to add an existing signature to this account, the Signature settings won't allow me to pick an existing signature (the list of signatures is empty). When I click on the '+' symbol at the bottom of the (empty) list of signatures, I can create a 'new' signature, but would rather have consistent signatures across all email account.

When I look at an existing email account that has a signature assigned to it, the list of signatures only shows the one currently assigned to that account, and I cannot select another existing signature.

Does anyone have any ideas on what I should be doing? Thanks.