Object of type 'FixLab.Components.Greeting' does not have a property matching the name 'Nam'. A component tag carries an attribute that the component does not declare as a parameter: usually a typo (here Nam for Name), a parameter that was renamed, or an HTML attribute such as class on a component that does not accept extra attributes. Correct the name on the tag.

If the component is meant to pass extra attributes through to its HTML element, give it a CaptureUnmatchedValues parameter instead, but read the warning in the fix section before you reach for it.

The error

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
      System.InvalidOperationException: Object of type 'FixLab.Components.Greeting' does not have a property matching the name 'Nam'.
         at Microsoft.AspNetCore.Components.Reflection.ComponentProperties.ThrowForUnknownIncomingParameterName(Type targetType, String parameterName)
         at Microsoft.AspNetCore.Components.Reflection.ComponentProperties.SetProperties(ParameterView& parameters, Object target)
         at Microsoft.AspNetCore.Components.ComponentBase.SetParametersAsync(ParameterView parameters)

The tag on the page:

<Greeting Nam="Maria Garcia" />

and Greeting.razor, the component it names:

<p>Hello, @Name!</p>

@code {
    [Parameter] public string? Name { get; set; }
}

Why it happens

The build does not catch it. With the typo in place, dotnet build reported 0 warnings and 0 errors, because Razor turns every attribute on a component tag into a named parameter value and leaves the matching for later. That happens when the page renders: ComponentBase.SetParametersAsync hands each name to ComponentProperties.SetProperties, which looks for a property with that name and a [Parameter] attribute. With no match and no catch-all parameter, it throws. The whole page returned a 500: one bad child tag stops the entire render.

A close cousin has its own wording. When the property exists but lacks the attribute, the message becomes Object of type 'FixLab.Components.GreetingNoAttr' has a property matching the name 'Name', but it does not have [Parameter], [CascadingParameter], or any other parameter-supplying attribute. Add [Parameter] to the property.

The fix

<Greeting Name="Maria Garcia" />

With the correct name the page returned 200 and rendered "Hello, Maria Garcia!". When a component really should forward extra attributes, such as a styled button or input wrapper, it declares a catch-all: 

<p @attributes="AdditionalAttributes">Hello, @Name!</p>

@code {
    [Parameter] public string? Name { get; set; }

    [Parameter(CaptureUnmatchedValues = true)]
    public Dictionary<string, object>? AdditionalAttributes { get; set; }
}

Then <GreetingLoose Name="David Chen" class="lead" /> rendered <p class="lead">Hello, David Chen!</p>. The warning: the catch-all swallows typos too. <GreetingLoose Nam="Maria Garcia" /> rendered <p Nam="Maria Garcia">Hello, !</p> with no error at all, the typo turned into an HTML attribute and the greeting left blank. Use it on components whose job is to pass attributes through, not as a way to make this error go away.

How it was reproduced

A fresh dotnet new blazor -o FixLab --interactivity Server project on .NET SDK 10.0.401 (ASP.NET Core runtime 10.0.12, no extra packages). A Greeting component with one Name parameter was used on a static page as <Greeting Nam="Maria Garcia" />. The project built cleanly, and a GET of the page returned 500 with the error above. The two variants, a property without [Parameter] and a component with CaptureUnmatchedValues, were run in the same project.

Frequently asked

What does 'does not have a property matching the name' mean in Blazor?
The component tag carries an attribute that the component does not declare as a parameter. Compare the spelling with the component's [Parameter] properties; a renamed parameter or an HTML attribute like class on a component without a catch-all causes it too.
Why doesn't the build catch a misspelled Blazor component parameter?
Razor compiles every attribute on a component tag into a named parameter value and does not report unknown names, so the build succeeds. The check happens at runtime, when the component receives its parameters, and the page fails with a 500.
Should I add CaptureUnmatchedValues to fix this error?
Only if the component is meant to forward extra HTML attributes such as class or aria-label to an element. It also swallows typos: a misspelled parameter becomes an HTML attribute and the real parameter stays empty, with no error.

More decoded errors in the Fixes category. Parameters from the beginning are in Blazor Components: Build Once, Use Everywhere.