Can the word "Cola" be used for commercial purposes?
It depends on what you want to do with it, but most uses would be permissible. "Cola" is, as you note, a generic term, comparable to "trout" or "sugar" or "beer."
Still, generic words may be subject to trademark protection. Even "coca" is a generic word. Such protection is, furthermore, not exclusive; trademark protection is more subtle and complex than most people realize.
There might be a company called "Cola" that operates guided outdoor activities, but that wouldn't necessarily prevent an unrelated company of the same name from pursuing the same business in another jurisdiction. Similarly, it might not prevent a company that repairs household appliances in the same jurisdiction from calling itself "Cola." For example, in the US, you can buy "Dove" personal care products and "Dove" chocolate; they are sold under distinct trademarks.
The central criterion is confusion: trademark protection primarily prevents others from confusing consumers about the source of goods or services. Even if a word or phrase is unambiguously a protected trademark, there are still allowable uses, most notably to designate the product, service, or company associated with the trademark. For example, if someone wants to tell a story about a bottle of Coca Cola (or even to claim "our product is better than Coca Cola"), the owner of the Coca Cola trademark cannot use trademark protection to prevent them from using the phrase "Coca Cola" in the story (or to make the claim).
For a specific opinion about a specific use of a specific trademark, you should engage a qualified lawyer with experience in trademark law.