The error message "The tag helper 'input' must not have C# in the element's attribute declaration area" indicates that there is C# code in an attribute of an 'input' tag, which is not allowed in ASP.NET Core. To resolve this, you need to remove the C# code from the attribute or use Razor syntax for the dynamic content. Refer to the Microsoft documentation for more information on tag helpers and Razor syntax in ASP.NET Core. As for the upgrade issue, you will need to contact the ASP.NET Maker support team for assistance.