Please help me to fix this error.

Simple: don't use Convert on user input.
Instead, use the appropriate TryParse methods:

decimal discountedCashPrice, quantity;
if (!decimal.TryParse(txtDiscountedCashPrice.Text.Trim(), out discountedCashPrice))
   {
   ... report problem to user
   return;
   }
if (!decimal.TryParse(txtQuantity.Text.Trim(), out quantity))
   {
   ... report problem to user
   return;
   }
decimal DisCp = discountedCashPrice * quantity;

The Convert methods always throw an exception if the user enters something wrong - and they do that, users, all the time - but the TryParse methods don't - they allow you to gracefully tell the user what the problem is, and let him fix it instead of crashing your app.