What are the biggest challenges for starting your own start-up online? what stops you?

Hello Farhan, In my opinion, it's always about the "idea" and what you can provide to your customers.
With today's digital world, anyone can have a website up and running but it's always about "how" and "what" you will be offering to your customers that other competitors can't.
The biggest challenge is coming up with that particular "idea" and setting milestones on how to achieve them.
The only obstacles than can stop you from launching your online business are failing to plan properly, no benchmark and lack of research and analysis.
If you know that you can serve your community with what they are looking for and help making their lives better, then your online business will be a success.
Just remember, planning is important.
Hope this helps and good luck!

whats important not the idea but how to execute the idea if you studied the market well and start planning you can start your business nothing will stop you