Your clue is in the error message:
KeyError: 'bpy_prop_collection[key]: key "Follow Track" not found'
Your code is accessing bpy.context.object.constraints["Follow Track"]
but it says your file cannot find any Follow Track Constraint
with the name "Follow Track"
. So one possibility is that it probably is named differently so you need to check the tracker's Object Constraint Properties
and adjust the name either in your script or in the field in red box marked as shown:

The other possible scenario is that there are other empties in your scene that are not trackers. So you need to further filter these out. They most likely do not have the FOLLOW_TRACK
constraint type so you should use this code:
import bpy
for tracker in bpy.data.objects:
if tracker.type != 'EMPTY':
continue
for c in tracker.constraints:
if c.type != 'FOLLOW_TRACK':
continue
bpy.context.view_layer.objects.active = tracker
bpy.context.object.constraints["Follow Track"].depth_object = bpy.data.objects["FBHead"]
Then make sure you have the object with name FBHead
because you are using that in your script bpy.data.objects["FBHead"]