Technically that isn't a cactus. It is a Euphorbia. Regardless of that, unfortunately it is rotting.

You can try getting a good knife and cutting off the top and bottom to remove the rotting part. The lines indicate about where you would cut. You need to remove the rotten tissue.

After cutting off the top let it sit for a couple days so the cut callouses and then try rerooting the top. You can find instructions for rooting cuttings on this forum. Discard the rotten middle part. The bottom part below the soil line might sprout a new branch from the side and continue growing.

Alternatively you can try cutting out the rot by cutting in a circle leaving you with a plant that has a hole in its side but I'm less confident that would work.

Some people here like to dust the cuts with cinnamon powder which seems to have some antimicrobial properties.

This is a Euphorbia plant so mind the sap that might ooze from the cuts. Some Euphorbia have very toxic sap and its best to not get it on your skin or any residue of the sap in your eyes. I'm not sure about this Euphorbia so best to be careful just in case.

Khansa, a week ago you posted two urgent requests for help with your plants. Several people took the trouble to reply, some at length, like Polypompholyx above, and you have not reacted, or thanked them in any way. Not very nice.

Dinie I couldn’t agree with you more. I’ve had this happen to me more than once and I always think “I couldn’t even get a thumbs up;)”. I try to give people the benefit of the doubt. Maybe something happened and the person forgot, but it happens enough where I don’t always think that’s the case. A simple thank you goes really far;).