Types referenced by serializable fields and properties must themselves be attributed with the [MessagePackObject].

[MessagePackObject]
public class A {
    [Key(0)]
    public B b;
}

public class B {
    public int Count;
}

Add the required attributes to the B class so that a dynamic formatter can be generated for it:

[MessagePackObject]
public class A {
    [Key(0)]
    public B b;
}

[MessagePackObject]
public class B {
    [Key(0)]
    public int Count;
}

An automated code fix is offered for this.

When B is formattable via a custom formatter, the diagnostic may be suppressed by declaring this attribute:

[assembly: MessagePackAssumedFormattable(typeof(B))]

When doing so, it becomes your responsibility to ensure that the custom formatter for B is discoverable via the IFormatterResolver object accessible through MessagePackSerializerOptions.Resolver.