I'm trying to connect the plugin to unity (with "add package from git URL")  but it shows this error:

[Package Manager Window] Error adding package: https://github.com/LootLocker/unity-sdk.git.

Unable to add package [https://github.com/LootLocker/unity-sdk.git]:

  No 'git' executable was found. Please install Git on your system then restart Unity and Unity Hub

UnityEditor.EditorApplication:Internal_CallUpdateFunctions ().

It sounds like you need to install git on your computer:

Git - Downloads (git-scm.com)

Otherwise, grab a .unitypackage file from here Release Lootlocker_UnitySDKv2.1.5 · lootlocker/unity-sdk · GitHub

Then just drop that into your project.

Notice those files are named for which version of Unity you're using