The manifest.json is setup incorrectly. It is trying to download "https://github.com/bvolvy/CustomLogoPlugin/releases/tag/v1.0.0" which is a webpage and not downloadable.
If you look at other plugins you will see that they point to zip folders containing a .dll file. This is what Jellyfin wants and needs to be able to install and run a plugin.
The manifest.json is setup incorrectly. It is trying to download "https://github.com/bvolvy/CustomLogoPlugin/releases/tag/v1.0.0" which is a webpage and not downloadable.
If you look at other plugins you will see that they point to zip folders containing a .dll file. This is what Jellyfin wants and needs to be able to install and run a plugin.