[Konstantinos]

As far as I know, this error comes when you've forwarded, but you haven't added the public callback.

pawn Код:

forward MathsWin(playerid);
forward ReactionWin(playerid);
forward NumberWin(playerid);

You need to add code, to the public callback that you don't have.

pawn Код:

public MathsWin(playerid)
{
    // something
    return something; // it depends 0/1
}
public ReactionWin(playerid)
{
    // something
    return something; // it depends 0/1
}
public NumberWin(playerid)
{
    // something
    return something; // it depends 0/1
}