Are you logged into the right account?

You could also try this if your sure you dont have more then one steam account;

https://psteamhelp.yuanyoumao.com/en/wizard/HelpWithGame
Enter name of the game in search bar,
Click on game,
Click on "it's not in my library",
"Restore removed package".

Look at your purchase history in your account management. If the game is there and hasn't been refunded or anything, then the game must be hidden or somesuch. If the game isn't in your purchase history than either you purchased it on alt account, or misremember purchasing it.

And before you dispute those options, think really carefully. Is it more likely your memory is flawed, or that Valve is randomly removing all traces of a game from your account, and only your account, and none of the other millions of accounts?

Well, he did post a review. I don't know this particular game -- can it be family shared?

引用自 Kargor：

Well, he did post a review. I don't know this particular game -- can it be family shared?

Yes. But you can not review a shared game.

It must be on the account they posted with.