Install it via
Sentry has extensive documentation for this SDK.
Check the Sentry Unity documentation for more details.
If you are using Unity 5.6 or 2018, these are not supported by this SDK. There are no plans to support versions of Unity older than 2019.4.
Sentry offered a minimalistic but stable SDK called sentry-unity-lite. It's available on a different repository. That SDK is now deprecated but is still compatible with Sentry server.