Do you plan on keeping this cactus inside? Cereus repandus get huge! Really are meant to be outside:).

Those marks look like frost or sun damage. There's nothing you can do about this. The cactus will heal itself. Looks like it already has. Keep an eye on it make sure it doesn't grow. If it continues to grow you could have other problems.

If this were mine I would get some perlite or pumice. Depending on where you live perlite may be easier to find. Perlite is cheap, and can be found at most garden centers. Get a bag of C&S soil. Do a mixture of 50% perlite or pumice with 50% C&S soil. When repotting try to get all the old soil off. Inspect the roots. Look for anything out of the ordinary, like bugs or rot. If everything is good repot with new mix. Do not water for a week after repotting. Goodluck:).

...and repot it much higher up in the pot as currently it's set way to low down inside...in what looks like a very un-cactus like soil mix :-)

And maybe in a clay pot. The pot on the photo looks shiny, plastic or some coated material which slows drying between watering, mostly if kept indoors.

Water well when dry. Judge when dry by appearance and the weight of the pot. It won't grow well if you water it too sparingly.