Value not saved while its appear in textbox

Sorry for that.
validation error: {"salesProductWeight":["The salesProductWeight field is required."]}

and this is the post code:


// POST: api/SalesItemLine
[HttpPost]
[Authorize]
public async Task<iactionresult> PostSalesItemLine([FromBody] SalesItemLine salesItemLine)
{
if (!ModelState.IsValid)
{
return BadRequest(ModelState);
}

SalesItem salesItem = await _context.SalesItem
.Where(x => x.Id.Equals(salesItemLine.SalesItemId)).FirstOrDefaultAsync();


salesItemLine.TotalAmount = (decimal)salesItemLine.Qty * salesItemLine.Price;

if (salesItemLine.SalesItemLineId == string.Empty)
{
var weight = _context.SalesProduct.Where(c => c.Id == salesItemLine.SalesItemId).Select(c => c.SalesProductWeight).FirstOrDefault();
var price = _context.SalesProduct.Where(c => c.Id == salesItemLine.SalesItemId).Select(c => c.SalesProductPrice).FirstOrDefault();
salesItemLine.SalesProductWeight = weight;
salesItemLine.Price = price;
salesItemLine.SalesItemLineId = Guid.NewGuid().ToString();
_context.SalesItemLine.Add(salesItemLine);
await _context.SaveChangesAsync();
return Json(new { success = true, message = "Add new data success." });
}
else
{
_context.Update(salesItemLine);
await _context.SaveChangesAsync();
return Json(new { success = true, message = "Edit data success." });
}

}