CS0246: The type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?)

Hi I am getting below error can you help me why

Compilation Error Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.  Compiler Error Message: CS0246: The type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?) Source Error:
Line 20: <namespaces> Line 21: <add namespac ...