This is very likely due to the macOS Big Sur issue - see this forum thread: Bug? Generated UnityLoader.js fails in iOS 14 public beta
That has been fixed in latest alpha, and patches should be landing to other Unity branches very soon.
Can you try out doing a build in Unity 2021.1 alpha? Or try out a workaround in the above thread? Doing a Development build should give a clearer callstack about what is failing.