[Burridge]

"idx" isn't a string. So it just needs to be "new idx;".

Code:

public OnPlayerCommandText(playerid, cmdtext[])
{
    if(strcmp(cmdtext, "/veh", true) == 0)
    {
        new String[128];
        new tmp[128];
        new idx;
        new Float:x, Float:y, Float:z;

        tmp = strtok(cmdtext, idx);
        if(!strlen(tmp))
        {
            return SendClientMessage(playerid, rosu, "You didn't give a vehicle name");
        }
        new vehicle = GetVehicleModelIDFromName(tmp);

        if(vehicle < 400 || vehicle > 611) return SendClientMessage(playerid, rosu, "That vehicle name was not found");