There's two possibilities, Joni:

If the tan areas are small, oval, raised spots, it might be scale insects. Those can be dealt with with a mixture of captan insecticide with a surfactant, such as Hi-Yield Spreader Sticker.
If they are medium to large, irregular, corky spots, sometimes slightly sunken in the center, It may just be a symptom of old age. The cactus will probably live for decades more, but the lower parts will continue to turn to "bark", as the oldest photosynthetic cells wear out. I'll admit that the plant seems a bit young to have such extensive areas of corkiness. Bad infestations of mealybug or cactus bug would leave scars like this, but the former would be obvious, and I have never seen the latter on indoor plants.

Thank you so much!! I move the plant outdoors for 8 months out of the year, so it might be scars. If you have a problem with bugs, do you know if you ever see the bugs? Thank you so much for your help!

Juvenile (they do the most damage) cactus bugs are <1/2" long and wide, shield-shaped bugs that scurry over the plant, usually most active and visible at dawn and dusk. The adults are about 1" long, and black with tan or russet speckles at the edge, and long black legs. Wherever they bite, they leave a circular yellowish spot, which later turns tan and corky. Once they reach the adult form, the only thing that kills them is two bricks. : ) The juveniles can be controlled with pyrethrin--when visible--and synthetic pyrethroids or carbaryl (not captan, like I mistakenly said above) can be used for residual action, if they are shy.