She screwed up by letting the Replicator of herself have access to the design of the weapon. Usually this type of screw up is done by Daniel Jackson. But after this happens everyone is telling her that she shouldn't feel bad as it wasn't her fault. Why?
A few relevant quotes from the episode:
"T'ealc, what have I done?" "You cannot be held responsible for the actions of the replicator."
"This isn't your fault."
"Carter, she isn't you." "O'Neil is correct, though she shared your memories, her personality was altered."
They're not trying to tell her that she didn't screw up by allowing the replicator access to the weapon. It's fairly obvious that she should feel lousy for letting an enemy get the drop on her like that.
They're trying to be tactful in saying that she shouldn't feel worse because it was a replicator version of herself. Carter is still empathizing with the replicator version, and may feel that because the replicator betrayed the SGC she is personally participated in that betrayal, beyond what she'd feel if it had been some other random bad guy who betrayed them in the exact same way.