Hi @albert9393
The cruise control system works in conjunction with the collision avoidance system so maybe because the avoidance system is showing an error it's causing both not to work.
You may need an appropriate scan tool to check what DTC codes are shown to find out what's wrong.
Here's an image showing the codes for the collision avoidance system from the 2015-2018 Mustang service manual that may be of some help.
[image|3671588]
(click on image)