Unfortunately, the latest version of Safari (14.0.3) has introduced an issue for Salesforce on Lab 3b, Step 36. When users click New Rule, they see an erroneous message about an internal server error and they can't continue to the next steps.
Until this is fixed, students can make a quick/easy change to Safari preferences that will fix the issue. The fix is now posted to the Help FAQ at MaxzPlace. (Alternatively, students can switch to Chrome for macOS. )
Sorry for the hassle. Hopefully, the next update of Safari will fix this!