Why am I getting this message when I try to save in FMB?
 

It's been a long time since I used FMB, and that was with the FB series. i'm just starting to learn again. I made and saved a couple of simple missions and then decided to make one a bit more complicated.
I just spent an hour or two placing objects in a mission and when I tried to save it I got this warning message:

chief '4_chief' contains only one waypoint. Did you forget to set NPC flag?

I have only 2 aircraft in the mission, and each has about 5 waypoints. Neither aircraft (or call sign) is called '4_chief'. The mission is not called that either.

I have a few trucks on the ground with waypoints, but I can't see that they should cause a problem.

Any ideas as to why it won't fly or save? What does the warning mean?

The warning means that you have an actor that has only one waypoint.

To find that actor use the Mission Objects List (View Menu).

I found it was a fuel truck. I don't know why it required a waypoint.

Now I have another issue though. I get this message when trying to save (and it won't save):

building '0_bld' has unlinked hooks

...and when I delete this building I get the same message for '5_bld' etc.

I'll soon have nothing left of my mission objects. I deleted all trucks and now the buildings are going too.

First if you use an actor like a truck you need a second waypoint, simple set one near the first or look into artillery they don't need a waypoint or into static object there also trucks you can use without need of a wp.

The building '0_bld' has unlinked hooks has to do with larger industrial buildings (these who use more than one part to finish), it's a bug we can nothing do against. Only solution is not to use them. :(

OK, thanks a lot for your help.
I'll save my work regularly during the build to save this sort of thing happening.

In the current release there are now factory objects under Static (No country)/Building/Industrial - these look identical to the German factory buildings but they can be assembled on the map without the unlinked hooks error.