asset() is simply a helper to generate a fully qualified URL to the path supplied, relative to the public/ folder in your application.
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The first thing to do is to verify that the actual asset is there (or else it won't show up).
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Then ensure the URL that gets generated is accessible from the place the mail gets read. IE if you're reading your mail @ Mailtrap (on the internet) then your image URL must resolve, publicly, on the internet. References to assets on your local storage, obviously, will not resolve.
Given that you are NOT using markdown email, perhaps what you are looking for is to embed the image in your email:
This may be achieved with this command:
<img src="{{ $message->embed($pathToFile) }}">