That feature isn't there (yet).
You need Lightroom Classic for that which is what Lightroom is now called
With the "new cloud" Lightroom I would expect that it will not have all the tools the "old folder" Lightroom has. It is being developed for the "online people" who will likely use it differently and expect different tools.
The "\" (backslash) key or a button on the lower right flips between them.