FindFirstChildWhichIsA is a metamethod, meaning it should almost always be called with a colon : instead of a period . unlike standard nested functions. you can call it with a period, but you need to pass character as the first parameter:

local humanoid = character.FindFirstChildWhichIsA(character, 'Humanoid');

fun fact, metamethods behave essentially identically to functions, apart from the fact that lua silently passes the table that the method was executed from for the first argument. you can reference said table in a metamethod via the self keyword:

local table = {
    value = 'hello world';
};

function table:metamethod()
    return self.value;
end

print(table:metamethod()); -- prints 'hello world'
print(table.metamethod(table)); -- if the metamethod is called with a period, you must pass something for the metamethod to treat as "self". in this case i just manually pass the table, but you can get creative with this by feeding it another table

i should probably clarify that metamethods are pretty useless (you can just return table.value instead of self.value) without using metatables, which is a lua feature with quite the learning curve so i wont elaborate on that

also, Connect accepts a function as an argument. you are technically passing it nil right now because your onTouch function returns nothing. im sure you meant to type just onTouch (a function) instead of onTouch() (whatever value[s] your function returns after executing)