Help Me Pls - With Database And Insert Statement

I Have an error:  Server Error in '/quanlythietbi' Application. INSERT statement conflicted with COLUMN FOREIGN KEY constraint 'FK_yeucau_nhanvien'. The conflict occurred in database 'equipment', table 'nhanvien', column 'manv'. The statement has been terminated.
Source Error:

Line 129:mycommand.Parameters.Add(new SqlParameter("@noidung_yc1",System.Data.SqlDbType.Text));
Line 130:mycommand.Parameters["@noidung_yc1"].Value = TextBox1.Text;
Line 131:int i = mycommand.ExecuteNonQuery();
Line 132:if (i>0)
Line 133:{
and this is my code:
string sqlstring = "Select * from yeucau where ngayGiaiQuyetxong='"+ Label8.Text +"' and date_yc='" + Label7.Text + "' and manv_yc='"+ TextBox2.Text + "' and noidung_yc='"+ TextBox1.Text+ "'";
myconnection = new SqlConnection(stringconn);
mycommand = new SqlCommand(sqlstring,myconnection);
myconnection.Close();
myconnection.Open();
mycommand = new SqlCommand(insertquery,myconnection);
 
mycommand.Parameters.Add(new SqlParameter("@ngayGiaiQuyetxong1",System.Data.SqlDbType.Char,10));
mycommand.Parameters["@ngayGiaiQuyetxong1"].Value = Label8.Text;
mycommand.Parameters.Add(new SqlParameter("@date_yc1",System.Data.SqlDbType.SmallDateTime));
mycommand.Parameters["@date_yc1"].Value = Label7.Text;
mycommand.Parameters.Add("@manv_yc1",System.Data.SqlDbType.Char,10);
mycommand.Parameters["@manv_yc1"].Value = TextBox2.Text;
mycommand.Parameters.Add(new SqlParameter("@noidung_yc1",System.Data.SqlDbType.Text));
mycommand.Parameters["@noidung_yc1"].Value = TextBox1.Text;
int i = mycommand.ExecuteNonQuery();
if (i>0)
{
lbcheck.Text = "Ä?ã Cập Nhật Yêu Cầu.";
}
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I don't know what I must do to repair it :(