Why did I get refused of entry into the UK even if I had a sponsorship letter? [closed]

I applied to volunteer at a scout activity center in London,Gilwell Park. I got refused of entry even with a sponsorship letter where it stated that I will have somewhere to.sleep. I would receive a weekly budget and it stated that I would return to.my country after 3 months of stay. Based on some paragraph 46A i got denied... The even took away my right for appeal... I'm a bit afraid to apply again because I don't know what I did wrong in my first application.