$dumpfile and $dumpvars not working in vscode error in terminal says requires system verilog

Question

Elaboration task '$dumpvars' requires SystemVerilog

is the error that is showing in the terminal when I execute

iverilog -o test_tb.vvp test_tb.v 

similar for $dumpvars

the codes are

//design

module test (a,b);

    input a;
    output b;
    assign b=a;

endmodule 

//testbench

`timescale 1ns/1ns
`include "test.v"

module test_tb ();

reg a;
wire b;

test uut(a,b);
$dumpfile("test_tb.vcd");
$dumpvars(0,test_tb);

initial begin

    a=0;
    #20;

    a=1;
    #20;

    a=0;
    #20;

    $display("test complete");
end   
endmodule

Answer 1

The system task calls need to be in an initial block like this:

initial begin  
  $dumpfile("test_tb.vcd");
  $dumpvars(0,test_tb);
end