What do you think of me not repaying my friend $6 for a cab ride?
Why wouldn't you repay your friend for the cab ride?
I spent the money on all of the things I paid for today and I don't have any money left.
I know that you may have already spent your money but you need to repay your friend for the cab ride. Your friend should not be expected to pay for your ride and it's not appropriate to not repay a friend when they lend you money.
It's not that I am unwilling to do it but I am not in a position to do it at this moment. I don't have any money.
It's understandable that you do not have money right now. However, you should explain this to your friend and let them know when they can expect to receive the money from you. Then pay your friend as soon as you receive the money.