Building a car is complicated. But you can divide it into smaller chunks of work to be done. That's the reason you can create an assembly line to build a car. Another property of a complicated system is it's predictability. If you want to make changes in the way the car is build, you can calculate or predict upfront what the impact on the whole system will be. The whole system is governed by known rules.

Our human body on the other hand, is a complex environment. If you and I both get Covid-19, and we both are treated with the same medication, we both - probably - will react in a different way. Complex systems are unpredictable, or at best not fully predictable. Human interactions are complex as well. That makes working in a team a complex environment. Not every body reacts the same way to eg. criticism. Product and Software Development are good examples of complex environments.

The problem is that many people - especially managers - still treat a complex environment as if it is a complicated environment. They still want to have very precise predictions and estimates on when the work will be done. They want to 'streamline' the process by keeping people busy 100% of the time. Many of the approaches that work in a complicated environment do not work in a complex environment. Even worse, often these approaches have a negative impact on the outcome and are slowing things down.