[[MWR]Blood]

pawn Code:

forward GMChange();
public GMChange()
{
    new rand = random(6);
    switch(rand)
    {
        case 0: SendRconCommand("changemode rivershell");
        case 1: SendRconCommand("changemode kiemas");
        case 2: SendRconCommand("changemode Helltunelis");
        case 3: SendRconCommand("changemode modas3");
        case 4: SendRconCommand("changemode usaandmec");
        case 5: SendRconCommand("changemode uostas");
    }
    return 1;
}

First of all, you do not need the playerid in the callback.
Second of all, you were defining 5 randoms, while you have got 6.
And probably you have missed the forward of the callback.