In reply to tyyang:
Make sure that your coverage class is declared before your scoreboard class. If it is and it’s still failing, try moving the instantiation to the constructor:
coverage cov;
function new(input mailbox drv2sb, input mailbox mon2sb);
this.cov = new();
this.rcv_from_drv = drv2sb;
this.rcv_from_mon = mon2sb;
endfunction
Maybe your simulator just gets confused by the syntax.