Upgraded from Alfred 4 to Alfred 5 and had this same problem on Monterey 12.5.1 (most recent).
I followed the official troubleshooting step 2.2 to Widen Search Scope and it worked immediately. https://www.alfredapp.com/help/troubleshooting/indexing/monterey/
It's the same as emirhan's advice but is missing the last step, to open finder and dropp in Macintosh HD to the search scope so only "/" appears.