People are willing to pay perfectly good money for a shirt with a Nike logo on it because an advertising firm here where I live turned Nike into a household name and built a connection in the mind of consumers between the Nike brand and great athletes. That took years and lots of money, not to mention talent and smarts on the part of Wieden and Kennedy (RIP Kennedy).
Without a solid connection to a rich rapper, your aspirational rich rapper brand is just that, aspirational. If you were a famous rapper, or had one on the payroll to wear your stuff on stage and in videos, you would have to hire people to count your merch money for you. Too poor and ordinary to become rich and famous is not a problem that can be solved with Facebook ads--or, in most cases, even be solved at all.
It is hard enough to succeed in selling what people are actually looking for: cute dog and cat shirts, shirts about a hobby some sub-section of society is passionate about like mountain biking or flying, dumb sayings about one thing or another that are given as gifts. Even these are difficult to succeed at because there is a lot out there for potential buyers to choose from, but at least people are actively looking for it and willing to pay for it.
You have no doubt seen brands similar to yours in style/theme that have succeeded. How did they do it? Why did people care about them and their brand? Were they already some kind of famous before they tried to sell shirts? Did they hire someone famous to wear their brand? Did they get incredibly lucky and have someone famous wear their stuff just out of the blue (if so, they should have bought a Lottery ticket while they were at it). What you haven't seen is all the countless people who tried to emulate that success without the requisite fame and fortune, and no one else saw them either.
All that said, best wishes in whatever you. Just be sure to take the time to learn from the mistakes and experiences of others, as well as your own. We pay for our mistakes, might as well get our money's worth.