You applied for a Standard Visitor Visa to attend an assessment session in the UK with the British Army. You are a student, are not working, have no dependants and entirely rely on your father to fund this trip.
Hate to break it to you, but at this point your chances of joining the British Army are slim.
The simple fact is that they want to see that you are able to support yourself while in the UK, without engaging in illegal employment. Almost invaribly, this has to be shown through several consecutive bank statements of your own. Furthermore, they want to see that you have family or financial ties making you able to say "this is why I have to go back home"
If you're serious about wanting to join the British Army, your father will have to pay for a qualified immigration solicitor. This website provides a good starting point. If you're lucky, they may tell you there is a way to secure a visa. In this case, co-operate with them and follow their advice scrupulously, because again, your current circumstances are not in your favour.
On the bright side: in another question you mentioned having three sisters who are British soldiers. Did you mention that when applying? If not, you definitely need to explain it to whatever solicitor you hire, and put it forth in the application in a very specific way that only a solicitor will be able to do correctly. If it becomes clear that being a soldier is a credible choice of career for you, your credibility as an applicant could massively increase. However, this is by no means a guarantee.