Add C# code to page

I have a list and I want to create a button on the page so that the contents of that list wil be exported to Excel.

I edited the .aspx page to add a button, but when I add C# code I get the error that code blocks are not allowed. The solution is to change the web.config file but that's not possible with SharePoint Online? Is there a solution for me? I'm not a pro in Visual Studio.