We should be able add/edit campaign tags (for custom reports) on emails that have already been sent.
By design, we do not allow campaigns that have been sent to be edited. While tagging a sent campaign retroactively may seem innocuous, the Dev Team will need to investigate potential impact if we were to relax this rule in this case.
I am not committing to allowing this feature pending investigation from the Dev Team.
Vice President - Product and Client Success
Maropost Marketing Cloud