Hi there, with the fundamental change of Optimizely CMS Version 12 to .net5 there are some differences in the configuration of Optimizely Search & Navigation (SaaS) Search Service. Here you can find a step by step introduction to get it up and running in your local development environment.
Create your developer index
Create your own developer index for testing first https://find.episerver.com/
Adding Search & Navigation to your website
Install latest NuGet package EPiServer.Find.Cms within your Solution
Install-Package EPiServer.Find.Cms
Configure Search & Navigation in your website
In your Startup.cs class add Search & Navigation in ConfigureServices method
services.AddFind();
In your appsettings.json add the find configuration with the values of your developer index

If you use external Identity Provider like Azure AD like in my case, please map the admin users to the virutal roles WebAdmins and SearchAdmins in order to get access to the Search & Navigation dashboard

Go to the scheduled jobs in the Admin area and execute job “EPiServer Find Content Indexing Job“, this one should complete successfully
That should be all for a simple integration of the Search service. Further information you can find also here https://world.optimizely.com/documentation/developer-guides/search-navigation/