Is the brown bit soft and moist?

Is that the plant's permanent growing condition, in that bucket-ish container? If so, it is potted far too low.

I am not a cactus grower, but I can tell you from reading here that the cactus needs to be in a container with holes in the bottom in a fast draining soil. Most often recommended here is commercial cactus/succulent potting mix combined 50/50 with perlite or pumice from your nursery. If you repot in that pot with the soil level a couple inches from the top, I believe it will be too much potting mix for the plant; it won't dry out fast enough.

Other cactus growers will check in here and advise you about the brown area, but I suspect you will have to start over. Personally, I prefer clay pots for these plants which don't like wet feet. Good luck with your plant.

Your container is way too big for the size of the plant. Being plastic, it takes long to dry already and all that extra soil means it takes even longer to dry. I would cut out the good parts and throw out any rotting parts, if a part of a pad is bad, throw out the bad part and keep the good part (as if you were preparing potatoes), the non-rotting pads and pieces of pads will root soon enough. Opuntia (rabbit ear cacti) are one of my favorites. I have the cold hardy Opuntia humifusa all over the place as ground cover and they are blooming now. Frustrate the he// out of the deer.

Thank you both. A few months ago I had to replant it in a bigger container because it got way too big for the one it was in and this bucket was all I had. At the bottom of the bucket there is rocks but since there's no holes I try not to water it too much or too often. But I'll cut off the pads and restart in a smaller container with drainage holes already in it. Thank you for your help!

Plastic pot with no hole is a "No-No" for this plant. Also your soil looks too rich and water retaining. I would repot in a different pot, soil and as suggested much higher. C&S soil mix with perlite or pumice 50/50 is best. However these will grow in pretty much anything if you can't get new soil. You just need to find some gravel or coarse sand to improve the drainage. If the brown spot is getting soft you definitely should do "surgery" to eliminate all rot.

Hello . My cactus is very old and unfortunately its browning process has increased. But my problem is that this disease is on the trunk of the plant. Do friends have a suggestion to fix my problem?