Whatever you are launching (i.e. a new business, product, service, course, promotion or book), you have to plan for it to give yourself the chance to be successful.
If a launch hasn't been successful it's not because what you're offering is rubbish - well, it might be but that's a different conversation - it's more likely because you've missed the principles of planning a launch:
Know your target market
Know you target sales
Know your timeline
Know your marketing content
But before you launch anything you need to know that there's a market for it, so test the water - see what people want and what you like to do because at the end of the day it's got to make you money.