Can u open a business, whilst still writing yr business plan?

There are many businesses that opened and ran for years without a written business plan. That being said, you really should have a written plan for your business at some point soon. The more important thing is that you understand and comply with all the legal requirements for your business (license, taxes, permits, etc.) prior to doing business. Failing to start with all your legal ducks in a row is very likely to cause you problems down the road.