The Failed to preview theme error message is the result of outdated or incorrectly formatted code in your Help Center theme.
To fix this issue
Export your custom theme. Don’t deactivate the theme while exporting it.
Fix the bad or corrupted code.
Import the theme back into your Help Center.
Warning: Do not deactivate or switch to another theme if you want to keep the corrupted theme as the live theme. If you deactivate or switch the theme, Guide prevents you from switching back to a corrupted theme.
Why do I receive a Failed to preview theme error when I try to preview my custom theme in Zendesk Guide?
The Failed to preview theme error message is the result of outdated or incorrectly formatted code in your Help Center theme.
To fix this issue
Warning: Do not deactivate or switch to another theme if you want to keep the corrupted theme as the live theme. If you deactivate or switch the theme, Guide prevents you from switching back to a corrupted theme.