Of course, add coolant. Generally, coolant is added by mixing half coolant and half water, and distilled water is needed. Don't add tap water because minerals in tap water will change the properties of coolant and make it unable to work normally. The adjusted coolant can be added from the water inlet of the water tank. When the coolant reaches above the lowest level of the water tank and below the highest level, it will be fine.
I hope my answer can help you.