Cannot drag & drop installer DMG files into applications

So, I recently updated my OS, and unusually this time, on rebooting I discovered that WhatsApp had been wholly removed - including the icon in the dock.

I assumed I just needed to update said software, so I went to the App Store and clicked download. All was good until a message appeared saying that the software could not be installed.

My second option was to go to WhatsApp's site and download from there. This was all good until I tried to drag the WhatsApp icon onto the Applications alias, where a ghosted-out, white icon appeared. (Which, bizarrely, I cannot capture with a screen-grab)

I can drag the icon anywhere but the applications folder desktop, downloads, One Drive, Dropbox, anywhere and it copies just fine. But no matter where I drag it successfully, I cannot then drag it to applications or the same ghosted-out white icon appears.

I have tried creating a new admin, same scenario. I have checked my Admin rights and they are all what I believe they should be.

So, what is/am I doing wrong?

OS 13.5.2

WhatsApp V2.23.17.82