Hi, could you provide more context? or code?
I tried to reproduce but I don't get the same exception.

To help you out, first, have you configured assets_directory in your configuration file?
By default, the assets are fetched from the assets folder.

Documentation on assets
Documentation on configuration

If you haven't set this up, you can fetch an image located in the public folder by doing this directly in your Twig template.

<img src="{{ gotenberg_asset('../public/img/logo.png') }}"/>

You can also check out this repository for an example implementation.