Safari not opening local html files after upgrading to macOS Catalina Beta I recently upgraded to macOS Catalina (10.15 Beta (19A546d)) on my Mid 2014 MacBook Pro. Then I can not open local html files on Safari (13.0 (15608.2.5)) and got the following error message. However, I can open websites on Safari and local html files on Chrome and Firefox. I don't have any third-party security software.
It turns out that Safari did not have permission to read my html files. Go to

System Preference &gt; Security &amp; Privacy &gt; Privacy &gt; Full Disk Access

and add safari. (Answer taken from comment.)