Hi,
Sorry to hear you are having problems getting previews working. This error can often indicate a crash in your application when it launches, or a problem with the simulator. If you look into the folder ~/Library/Logs/DiagnosticReports, do you see any crash reports for your app, anything previews or sim related?
If nothing stands out to you, then best next step will be to file a feedback with diagnostics so we can take a look. Feedbacks support huge files, so the diagnostics should upload fine there.
We will need the diagnostics Xcode Previews generates in order to make sure we understand the error the previews system is encountering.
If you are having issues with Xcode, macOS, catalyst or on-device iOS previews then download and install the logging profile for your device. Instructions and profiles are available here: https://developer.apple.com/bug-reporting/profiles-and-logs/?name=swift
Attach the sysdiagnose to the radar as well as the diagnostics using the instructions below.
Then when you reproduce the problem in Xcode:
- Either (a) an error banner will appear, click the "Diagnostics" button in that banner; or (b) if you're not seeing an error but you still want to provide diagnostics you can get the same diagnostics window by going under the Editor menu in the menu bar, then selecting the Canvas submenu, then selecting "Diagnostics".
- In the sheet that appears, click "Generate Report" in the bottom left of the sheet
- Attach (or make from the folder) the resulting zip file to the bug (will be named something like previews-diagnostics-0123456789.zip)
- Generate the sysdiagnose(s) and attach those too
