トレイルヘッド
Check if the New button is hidden
https://automationchampion.com/2022/02/27/how-to-hide-the-standard-new-button-from-list-view/
The answer did not really help me. I solved the issue by going out of the lightning GUI and using the old one. Then, I could see the Reservation object.
Go to Object Manager-->Cases-->Record Type-->Product Support--> Under Picklists Available for Editing Click edit next to Type-->Add Problem to Selected Values--> Create a new case with Type Problem.
https://trailhead.salesforce.com/trailblazer-community/feed/0D54S00000BuSCk
I don't think it's necessary unless there's a challenge or module advocating for installing a package.