If you are getting an error saying that you cannot preview this file (while using SwiftUI), then perhaps like my experience, a reboot might fix it. That’s the short version to possibly get you going before reading on.
I created a new project today and switched the default ContentView.swift to use an iPad Pro 11 inch for the preview. An error appeared in right pane saying “Cannot preview in this file – Unable to boot device due to insufficient system resources”. Clicking the diagnostics button provided the following:
HumanReadableNSError: Unable to boot device due to insufficient system resources.
Please see Simulator Help for information on adjusting resource limits.
==NSLocalizedFailureReason: The current system settings are not sufficient to allow booting additional simulators: maxFiles: 12288, openFiles: 11669, enforcedFilesBuffer: 1868
As mentioned, a reboot fixed the issue. It seems that Apple last recognised this happening back in 2017 in the Simulator Help section on the Apple help site that gives some recommendations on how to modify the number of processes and number of open files.