This is happening most of times, no need to worry we can fix with one of the following solutions.

Solution 1:

Go to Xcode preferences -> Locations -> Command line tools

Here, most of them are set to Xcode 10, change to Xcode 11.

Restart Xcode 11 beta, the try it should be ok.

Solution 2:

1. Delete the ContentView.swift file.

2. Create new 'SwiftUI View'

3. Name it same as ContentView.swift.

4. Now click resume in the canvas, we can see preview loading.

Sometimes restarting system may also help.