Tall cactus

As something of a cactus expert (two large greenhouses full of them), I can tell you that it is Cleistocactus straussii, and the browning and loss of spines at the base is normal, just ageing.
It should be potted in a very gritty mix (John Innes 2 or 3, plus fine grit @ about 50/50. Don’t let it stand in water after watering, and give it the sunniest windowsill you have, to prevent it stretching (called etiolation).

Could it be rotting at soil level due to overwatering?
Did you use potting compost when repotting? It holds water so is not suitable for cacti.

I think the main thing about growing cacti is to keep close to a sunny window and avoid over-watering. If it’s too far from bright window it may be more inclined to stretch up and get lanky.