Q:

The relative page path 'About' can only be used while executing a Razor Page. Specify a

First of all I have searched about this error and found nothing. The full exception:
    System.InvalidOperationException: The relative page path 'About' can only be used while executing a Razor Page. Specify a root relative path with a leading '/' to generate a URL outside of a Razor Page.
   at Microsoft.AspNetCore.Mvc.UrlHelperExtensions.CalculatePageName(ActionContext actionContext, String pageName)
   at Microsoft.AspNetCore.Mvc.UrlHelperExtensions.Page(IUrlHelper urlHelper, String pageName, String pageHandler, Object values, String protocol, String host, String fragment)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.DefaultHtmlGenerator.GeneratePageLink(ViewContext viewContext, String linkText, String pageName, String pageHandler, String protocol, String hostname, String fragment, Object routeValues, Object htmlAttributes)
   at Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Process(TagHelperContext context, TagHelperOutput output)
   at Microsoft.AspNetCore.Razor.TagHelpers.TagHelper.ProcessAsync(TagHelperContext context, TagHelperOutput output)
   at Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner.RunAsync(TagHelperExecutionContext executionContext)
   at AspNetCore.Views_Shared__FooterPartial.ExecuteAsync()
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, Boolean invokeViewStarts)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context)
   at Microsoft.AspNetCore.Mvc.TagHelpers.PartialTagHelper.RenderPartialViewAsync(TextWriter writer, Object model)
   at Microsoft.AspNetCore.Mvc.TagHelpers.PartialTagHelper.ProcessAsync(TagHelperContext context, TagHelperOutput output)
   at Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner.RunAsync(TagHelperExecutionContext executionContext)
   at AspNetCore.Views_Shared__Layout.<ExecuteAsync>b__40_1() in D:\Documents\Obaju\Obaju.App\Views\Shared\_Layout.cshtml:line 40
   at Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperExecutionContext.SetOutputContentAsync()
   at AspNetCore.Views_Shared__Layout.ExecuteAsync()
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, Boolean invokeViewStarts)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderLayoutAsync(ViewContext context, ViewBufferTextWriter bodyWriter)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(ViewContext viewContext, String contentType, Nullable`1 statusCode)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(ActionContext actionContext, IView view, ViewDataDictionary viewData, ITempDataDictionary tempData, String contentType, Nullable`1 statusCode)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.ViewResultExecutor.ExecuteAsync(ActionContext context, ViewResult result)
   at Microsoft.AspNetCore.Mvc.ViewResult.ExecuteResultAsync(ActionContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeResultAsync(IActionResult result)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResultFilterAsync[TFilter,TFilterAsync]()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResultExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeResultFilters()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
   at Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.MigrationsEndPointMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

About is Razor Page accesed via /about url path and it works well. This error happens only when I try to go on /home/index or another mvc view.

A:

The Problem
In one of your Razor Views, there is an anchor tag that looks something like this:
<a asp-page="About">About</a>

That syntax (without the forward slash /) will only work inside a Razor Page. 
The Solution
Inside a Razor View, prefix the Page name with a forward slash / like this:
<a asp-page="/About">About</a>

Distinguishing Razor Views from Razor Pages

Razor Views are usually in a Views directory. 
Razor Pages are usually in a Pages directory and have an @page directive at the top of the file.

A:

<a class="nav-link" asp-area="Identity" asp-page="/Account/Manage/ChangePassword">
For creating Razor Page link, add asp-area and then asp-page.

