I'm guessing that you clicked on the blue button at the BOTTOM of the screen labelled "Continue to: Burning Studio 19". (That does, indeed, take you to a product that you have to pay for). If so, UNINSTALL the program you've got and RESTART your computer.
Then RETURN to my link above and click on the blue "Download" box at the TOP RIGHT of the page. That gets you the free program.
It's a long time since I last installed Ashampoo Burning Studio Free onto a machine but I'm guessing that you've been email an activation code and that you're trying to enter it manually into the program. That's never a good idea as it's easy to go wrong (e.g. by mistaking a number '0' for a letter 'O'). You need to copy and paste the code.
So you need to go to the email with the code in it and, while holding the left mouse button down, drag your cursor over the code to highlight it. (NB: Make sure that you ONLY drag over the code itself and that you DON'T highlight either of the spaces at the end of it). RIGHT-click and select 'Copy'.
Then go to where you need to enter the code. RIGHT-click in there and select 'Paste'. You should then have a valid code.