UVM Sequence Help

Hi all,

I am experiencing a problem with sequence/sequencer. I think that the problem is inside the sequence

class Sequence0 extends uvm_sequence #(Packet);

  function new(string name = "Sequence0");
		super.new(name);
		`uvm_info(get_full_name(), "Sequence new()",UVM_LOW);
	endfunction : new

	Packet item;
   
	 	
  `uvm_sequence_utils(Sequence0, Sequencer) 
        
	
	virtual task body();
    forever begin
		    `uvm_info(get_full_name(), "Sequence build()",UVM_LOW);           
			`uvm_do_with(item,{da.size == 6;}); 
                     
  	end
	endtask : body
	
endclass : Sequence0

The compilation works; on the other side the run phase shows a

"UVM_FATAL ./sequence_macsec_tx.sv(11) @ 0: uvm_test_top.t_env.Seqncr@@Sequence0 [DCLPSQ] \$unit ::\Sequence0::m_set_p_sequencer  uvm_test_top.t_env.Seqncr.Sequence0 Error casting p_sequencer, please verify that this sequence/sequence item is intended to execute on this type of sequencer"

I inserted on purpose the `uvm_info and the last message that I get is Sequence new() as stated in line 5. 

Do you have any idea about the above error? 

Thanks in advance