-salvagewallet is not a command that is run from the debug console. It is not one of Bitcoin Core's RPC calls. Rather -salvagewallet is a command line option. You set it in the command that is used to run Bitcoin Core or you set it in the bitcoin.conf file by adding salvagewallet=1 to the bitcoin.conf file and then starting Bitcoin Core.

Note that -salvagewallet does not always work and sometimes may result in corruption of your wallet if it is not already corrupted. It is only as a last resort option. You should always make a backup of your wallet file prior to starting Bitcoin Core with -salvagewallet.