Same problem here. Users have "Send As", "Send on behalf of", and "Full Access" to a user account that is being used as a shared mailbox. When trying to approve from the shared user's mailbox they get the same error message. One solution on a different post was to get them to sign-in as the shared user instead of just accessing the user's mailbox from their own Outlook. I find that solution not feasible as they can read and respond as that user but if they want to approve they need to sign-out of their account, then sign-in as the shared account, make the approval, and then sign-out as the shared, then sign-in as their own account. Way too many steps.
The https://learn.microsoft.com/en-us/outlook/actionable-messages/actionable-messages-faq explicitly says that shared or group mailboxes cannot be used for the actionable approvals, which is why I was trying to use a user's mailbox that is shared via delegate permissions as this seems the only supported way, but alas we get "Failed to validate the signature of the actionable message card."
I asked CoPilot about the approvals and it firmly stated having "Full Access" was sufficient to make approvals on behalf of another user account.
If you did find a solution, I would appreciate hearing it!
