As I reported in previous message You should uncomment
folder-hook "imap.gmail.com" fcc-hook .....
in Your .muttrc file, if You of course had it defined at all.
But it is possible to use fcc-hook in the following way
fcc-hook *.* "=Sent"
In this case mutt will write the copy the text of the message,
with attachments, into the local file /Mail/Sent (I use root user).
I do not set it because gmail makes the copy of sent files on the server.