You shouldn't be calling .ToString().
As the error message clearly states, you're writing a conditional in which one half is an IHtmlString and the other half is a string.
That doesn't make sense, since the compiler doesn't know what type the entire expression should be.
There is never a reason to call Html.Raw(...).ToString().
Html.Raw returns an HtmlString instance that wraps the original string.
The Razor page output knows not to escape HtmlString instances.
However, calling HtmlString.ToString() just returns the original string value again; it doesn't accomplish anything.