Weaknesses are always tricky to talk about in your essays and recommendations. What the adcom is looking for is a weakness that is addressable and is not a red flag. A good example of this type of weakness would be that you sometimes do too much work on a project and need to learn to supervise other people better. Stay away from things like "being a perfectionist" which is not a real weakness and definitely avoid red flag weaknesses like losing your temper or missing deadlines.