Post App Installation, you need to configure remote site settings, permission sets, and Lightning Web Security. Follow the Docs  Metadata Search: Post Upgrade Steps and complete the configuration on your org.

Then the “Page Doesn’t Exist” error will resolve, and you can access List All Button.

Now check the List All option, Open the Metadate Search tab, and click on any of the List All 

In the new window will get all records of the metadata, like the below image.