In Odoo 16, when you create a rounding method with the strategy "Add a line", it will add a new line to the invoice to account for the rounding amount. If you do not see the line on the invoice, there could be a few reasons why this is happening.

1. Make sure that the rounding method is correctly set on the invoice. Go to the "Invoicing" module and select "Customers" or "Vendors" from the drop-down menu. Then, click on the invoice and go to the "Other Information" tab. Check that the correct rounding method is selected.

2. Make sure that the rounding line is not being filtered out. Go to the "Invoicing" module and select "Customers" or "Vendors" from the drop-down menu. Then, click on the invoice and go to the "Lines" tab. Check that the "Show Rounded Lines" option is selected.

3. Check that the rounding method is correctly configured. Go to the "Accounting" module, select "Configuration" from the drop-down menu and then select "Settings". Check that the Rounding method is correctly defined and selected.

4. Verify that the rounding is being calculated correctly. Go to the "Accounting" module, select "Reports" from the drop-down menu and then select "Rounding Entries". Check that the rounding entries are correctly calculated for the invoice.

If none of these solutions work, you can check the server logs for error messages that might indicate the problem

Thanks Ahmad for your answers.
but I checked everything and no problem only that when I open an invoice in the invoice lines tab I don't see the option to display rounding lines