Opening a XAML (Extensible Application Markup Language) file using the Visual Studio Designer view can result in the following error in ArcGIS Pro version 2.8 and above. The error message is often accompanied by various warnings, has no error code, and reads as follows:
Error: Could not load file or assembly 'ArcGIS.Desktop.Framework, Culture=neutral' or one of its dependencies. The system cannot find the file specified.
This error can occur when opening a XAML file in the Visual Studio 2017 and 2019 XAML Designer view in ArcGIS Pro version 2.8 and above add-in and configuration projects. The XAML files that produce the error message typically contain a reference to the Pro API, which is usually included with Pro SDK item templates in the form of a xmlns attribute line item such as:
Whereas previous ArcGIS Pro assemblies were built with the AnyCPU platform target, at ArcGIS Pro 2.8 and above, many of the assemblies are now built using the 64-bit platform target. The XAML Designer that ships with Visual Studio 2017 and 2019 is not capable of loading x64 assemblies. This causes the error message.
The following entry on the Microsoft Developer Community support website describes this Visual Studio limitation: Microsoft Developer Community: XAML Designer does not display x64 User Controls from external projects.
The error is related to the Visual Studio Designer only. It is not a compilation error and has no effect on building and running an ArcGIS Pro extension.