One very concise answer is market for Byju's paid online education is about 2 order of magnitude smaller than projected to investors.

It is for simple reason that paying capacity of average Indian customers is much lower than west and ton of cheaper options are already available for this kind of thing.

Detailed reasons are of course bad management, underhanded sales tactics, burnout, online vs classroom education, scaling too fast etc..etc.

High margin why?

Their online pre-recorded courses cost 1000s of dollars.

I can imagine that's never going to work.

Very few people can afford it, and those who can afford want more, e.g. Q&A, graded assignments etc. It's almost never a "record these once and we are done" thing.