Helps D: cant see/select gamemodes!

Hey, as the title says, when im making a game for me and my bro to play, I cant select a gamemode… i click the wrench on the right hand side, and go to where it says “Sandbox” but it wont let me open the drop down menu… anyone have any idea how to fix this??

Do you have another gamemode installed?

There are those default gamemodes like TTT and MelonRacer that come with Garry’s Mod