There are a few ways to install the unity plugin.

If you want to add the package though the asset store you can grab the github url and then paste it into the package manager in unity

You can also add the package directly to your manifest.json file located under Packages/manifest.json.

{
    "dependencies": {
        "com.unity.xrplc": "git+https://github.com/Transia-RnD/xrpl.csharp.unity",
    }
}

For more indepth instructions on installing see https://docs.unity3d.com/Packages/[email protected]/manual/index.html