Hi Karolin, 

Which version year of Sage 50 CA are you using? This article updated August 2021, on how to create and convert Sales Orders and Quotes into Sales Invoices attempts to explain the conversion process:

How to Convert a Quote/Estimate to an Invoice or Order

1. In the Home Window, click Customers & Sales (or Receivables depending on the terminology of your file).
2. In the Task pane, right-click the Sales Quote icon and select Find Quote from the menu.
3. Select the transaction you want to convert and click Select.
4. Click the Convert button
   • Select Convet to Invoice to convert into an invoice
   • Select Convert to Contract to convert into a contract
5. Enter the number of items you are selling in the Quantity column. To fill all the items on the order click  in the toolbar.
   • If you are partially filling the order and want to cancel the back order for any remaining items, click  in the toolbar.
6. Click either Process or Print & Process.

How to Convert a Contract to an Invoice

1. In the Home window, click Customers & Sales (or Receivables depending on the terminology of your file).
2. In the Task pane, right-click the Contracts icon and select Find Contract from the menu.
   • Or in the Order/Quote Number box, click  if you already have the window open
3. Select the transaction you want to convert and click Select.
4. Click the Convert button and select Convert to Invoice.
5. Enter the number of items you are selling in the Quantity column. To fill all the items on the order click  in the toolbar.
   • If you are partially filling the order and want to cancel the back order for any remaining items, click  in the toolbar.
6. Click either Process or Print & Process.

NOTE: Please note that a journal entry will not be posted as an Estimate or a Contract until it has been converted into an Invoice

Hope this helps!

Warm Regards, 
Erzsi