It must be Player, but not necessarily a variable named "player" (which was nil in your case, because you never told your script what it is). Both of your functions are executed by some events, and all events have a source (which is also a special variable available in scope of the function that gets executed by event). If you open the documentation page on wiki for onPlayerWasted you can see that in this case the source of event is some Player.
There is plenty of information on wiki, so you just need to learn how to use it properly. Follow the links and read everything which is related to the functions or events you want to use. Start by clicking on "source" link and you will see all information about event system Then check some Lua tutorials to learn about variables, scopes, functions etc., there are links on main wiki page too (https://wiki.multitheftauto.com/wiki/Main_Page, General Lua Help section).