Multiple build error

gistfile1.txt

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS0246	The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	3	Active
Error	CS0246	The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	1	Active
Error	CS0246	The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	2	Active
Error	CS0246	The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	4	Active
Error	CS0246	The type or namespace name 'Xamarin' could not be found (are you missing a using directive or an assembly reference?)	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	6	Active
Error	CS0246	The type or namespace name 'Application' could not be found (are you missing a using directive or an assembly reference?)	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	10	Active
Error	CS0518	Predefined type 'System.Void' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	12	Active
Error	CS0518	Predefined type 'System.Void' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	19	Active
Error	CS0115	'App.OnStart()': no suitable method found to override	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	19	Active
Error	CS0518	Predefined type 'System.Void' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	24	Active
Error	CS0115	'App.OnSleep()': no suitable method found to override	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	24	Active
Error	CS0518	Predefined type 'System.Void' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	29	Active
Error	CS0115	'App.OnResume()': no suitable method found to override	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	29	Active
Error	CS0518	Predefined type 'System.Object' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	14	Active
Error	CS0103	The name 'InitializeComponent' does not exist in the current context	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	14	Active
Error	CS0518	Predefined type 'System.Object' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	16	Active
Error	CS0118	'MainPage' is a type but is used like a variable	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	16	Active
Error	CS0518	Predefined type 'System.Object' is not defined or imported	App8.UWP	C:\Users\User\Source\Repos\App8\App8\App8\App.xaml.cs	16	Active
Warning	IDE0006	Error encountered while loading the project. Some project features, such as full solution analysis for the failed project and projects that depend on it, have been disabled.	App8.Android		1	Active
Warning		Error: No resource found that matches the given name: attr 'windowNoTitle'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	2	
Warning		Error retrieving parent for item: No resource found that matches the given name 'Theme.AppCompat.Light.DarkActionBar'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	2	
Warning		Error: No resource found that matches the given name: attr 'colorPrimary'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	2	
Warning		Error: No resource found that matches the given name: attr 'colorPrimaryDark'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	2	
Warning		Error: No resource found that matches the given name: attr 'windowActionBar'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	2	
Warning		Error: No resource found that matches the given name: attr 'colorAccent'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	3	
Warning		Error: No resource found that matches the given name: attr 'colorAccent'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	4	
Warning		Error retrieving parent for item: No resource found that matches the given name 'Theme.AppCompat.Light.Dialog'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	4	
Warning		Error: No resource found that matches the given name: attr 'windowActionModeOverlay'.	App8.Android	C:\Users\User\Source\Repos\App8\App8\App8.Android\Resources\values\styles.xml	4