I'll stop you here too. After this point, you are actually evaluating the sum and getting a closed expression for it. You should NOT continue to put in the summation signs as you aren't going to be summing anything after this point...
So once you have fixed everything, you should have
so, I'm basically asked to provide
as a function of n, without summation symbols, and for this I entered step i) , but it was considered as wrong, so obviously, my mistake is somewhere above,
, here I obtained , but it is actually wrong since all my work-up before is wrong,
can you, please help me with this,
which you can now simplify as much as you like