If the leaves on your cordyline are turning a crispy brown then underwatering might be the culprit, but if they’re turning dark brown and mushy then it might be overwatering.
Water when the top 25-50% of soil is dry. These plants like to be kept somewhat moist, but never soggy. Water thoroughly until it flows out of the drainage hole and discard any excess water.
Yellowing and browning on the leaves can be caused by the tap water used to water your plant. Normal tap water can have a lot of things like added minerals and fluoride in it that can affect sensitive plants like this. If possible, use distilled or rainwater. If this isn’t possible you can leave some tap water out overnight to evaporate some of the chemicals. Even if you’re using filtered water, I would check with the brand’s manufacturer to see if they filter out fluoride. Most do not!
These plants are native to the warm tropics, so they appreciate a warm humid environment. Mist often, add a pebble tray, or place a humidifier nearby.