reg qb; cannot be driven by primitives or continuous assignment

I'm trying to make an SR flipflop on Icarus Verilog.

Flipflop module:

module srff(sr,clk,q,qb);
    input [1:0]sr;
    input clk;
    output reg q,qb;

    always@(posedge clk) begin
        case(sr)
        2'b00: q=q;
        2'b01: q=0;
        2'b10: q=1;
        2'b11: q=1'bz;
    endcase
    end
    assign qb=~q;
endmodule   

Testbench:

module sr_ff_test;
    reg [1:0]sr;
    reg clk;
    wire q,qbar;

    srff sr_ff_test(sr,clk,q,qbar);

    initial begin $dumpfile("dump1.vcd");
    $dumpvars(0,sr_ff_test);
    end

    initial begin
    $monitor("S=%d, R=%d, CLK=%d, Q=%d, Qbar=%d",sr[0],sr[1],clk,q,qbar);

    sr[0]=1'b0;
    sr[1]=1'b1;
    clk=1;
 
    #100
    sr[0]=1'b0;
    sr[1]=1'b1;
     
    #100
    sr[0]=1'b0;
    sr[1]=1'b0;
     
    #100
    sr[0]=1'b1;
    sr[1]=1'b1;
     
    #100
    sr[0]=1'b0;
    sr[1]=1'b1;
     
    #100    
    sr[0]=1'b0;
    sr[1]=1'b1;
    end
endmodule

The error is as given in the question (line 14 of testbench - the one with monitor...). What went wrong, and how do I fix this?

I'm new to Icarus Verilog, and I don't know if I have accidentally used commands that aren't usable.

The error is in the srff module. You declared qb as a reg, but then you try to drive it with a continuous assignment using the assign keyword.

The solution is to not declare it as a reg. Change:

    output reg q,qb;

to:

    output reg q;
    output qb;