You have 2 options.

1, go find your server.vdf in GarrysMod/garrysmod/cfg and manually change the values there. Will have to reload server or change maps after changing this directly.

2, enter the commands like:
"slashers_duration_addobj" 120
instead of
"slashers_duration_addobj" = "120"

using the 'instead of' version, results in the variable being set literally as "="120" ", so its rendered invalid. Using the command like I showed above will result in what you would expect.