Natalie is 100% correct about this. I would add that cohesion is (arguably) necessary for coherence. I think this is one place where confusion comes from when people are preparing for IELTS. The two concepts are very closely linked. Without cohesion, there is no "flow" to your writing, so the coherence level goes down. Personally I think the choice of IELTS in making "Coherence and Cohesion" one category for marking is a little confusing. I would have just called it "coherence."

Coherence is about the big picture of your writing making sense, while cohesion is about the little connections that tie your sentences and ideas together. Both are important for clear, effective communication. Coherence: This is about the overall sense and flow of your writing. It means your ideas are organized and make sense together. Imagine telling a story that has a clear beginning, middle, and end. Cohesion: This is more about the glue that holds your sentences and paragraphs together. It’s the little words and phrases that link everything, like “and,” “but,” “then,” and so on.