data is not convert to string

Question

I am trying to convert the id to sting from the drop down. but the data is not saved in the database but the sending the mail is ok. the below is the format

var name = Convert.ToString(Session["Name"]);

var leave_type = ddlLeavetype.SelectedItem.Text.ToString();

// var leave_type = ddlLeavetype.SelectedValue.ToString();

var fromdt = _ut.ConvertDate(txtformdate.Text.Trim().Replace("'", "''"));

var todat = _ut.ConvertDate(txttodate.Text.Trim().Replace("'", "''"));

var reason = txtreasons.Text.Trim().Replace("'", "''");

var approver = ddlline.SelectedValue.ToString();

string sql = "insert into [dbo].[tbl_leave_apply]" +

" ([leave_id] " +

" ,[from_date] " +

" ,[to_date] " +

" ,[Reason] " +

" ,[approver_id] " +

" ) " +

" VALUES " +

" ('" + leave_type + "' " +

" ,'" + fromdt + "' " +

" ,'" + todat + "' " +

" ,'" + reason + "' " +

" ,'" + approver + "' " +

" ) ";

int i = _ut.ExecuteSql(sql);

but when i have changed the

var leave_type = ddlLeavetype.SelectedValue.ToString();

the data is not saved and the mail is taking the dropdown id. when i debug the code. the error

insert into [dbo].[tbl_leave_apply] ([leave_id],[from_date],[to_date] ,[Reason] ,[approver_id] ) VALUES ('PL','08/16/2019' ,'09/26/2019' ,'dasd','51840')

Msg 245, Level 16, State 1, Line 6

Conversion failed when converting the varchar value 'PL' to data type int.