$rose, $fell not accepted in iff condition?

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4

You could make this much simpler if you just called the sample method inside your always block. This would also improve performance.

covergroup count_active_inactive;
  inactive: coverpoint counter iff (sig) {bins cycles[] = {1,2,3,[4:$]};}
  active:   coverpoint counter iff (!sig) {bins cycles[] = {1,2,3,[4:$]};}
endgroup

count_active_inactive cai = new();
always @(posedge clk) begin
  if ($stable(sig))
    counter <= counter + 1;
  else begin
      cai.sample(); 
      counter <= 1;
    end
end