Why do I have red structures in my layout? How can I change that?

These red structures are potential structures that will not be installed due to some limitations in the electrical configuration. If it’s due to DC/AC ratio, please try enabling the “install the maximum peak power” button found in the “Power requirements” section of the “Layout” tab.  If it's due to a very small area, then you can try changing the number of inverters per power station, having the PS inside the DC field, changing structure and its configurations, decreasing the pitch distance and other setbacks, and still enabling the “install the maximum pick power”. Otherwise, you may want to use the “Group area in LV” feature. You can learn more about this feature in this article
It may also be due to you using the “Specific capacity” option. In this case, pvDesign will install the structures needed to reach your target capacity and the rest of structures will be highlighted in red.