Mac desktop says "Unable to download App" but the App is downloaded on the iOS device?
I'm on macOS 11.3 Beta (20E5172i) and have the same issue. I've bought an app on my iPhone and my MacBook appears to subsequently try to download that app. I get these messages whenever I download a new app on my iPhone.

I found this setting in the macOS App Store Preferences and disabled the automatic app download, but don't know yet if this fixes the issue.