You are getting the error "You cannot perform this action. Add-ins are not supported in this folder." when you are trying to open the Outlook server-side add-in in a shared mailbox.
SAP Cloud for Customer
Shared mailboxes are not supported by Microsoft Office JS API.
This is expected behaviour.
error; shared mailbox; outlook; server-side; add-in; , KBA , LOD-CRM-GW-SCC , Invisible CRM - Smart Cloud Connect Solution , Problem