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”.
It may also be due to you using the “Specific capacity” option. In this case, RatedPower will install the structures needed to reach your target capacity and the rest of structures will be highlighted in red.
In case you have multiple areas and are noticing red structures in some of them, you can use the "Group areas in LV" feature. You can learn more about this feature in this article. In some cases, even when using the aforementioned option, red structures may still persist in the design. Such cases may occur when there aren't enough structures to fulfil the minimum requirements of a string box or a string inverter within the same area. Basically, string cables are not allowed to cross from one area to another. To solve this, swap the areas forming the group by a bigger area that encompasses all of them and make sure to define restricted zones (RA polygons) between them. This should help install the maximum possible capacity