player names are case sensitive

You can also use /list to see a list of players, and next to those players will be a $player[i] identifier that you can use in place of their name if their name has special characters/is tough to type.

Making a mod can be a lot of work, but there's a guide here: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1570801575 , and there's also a discord community in the "where can I get more help" section of that guide dedicated to modding aground.