This is not a bug.

It is because of the game design.  A transport requires a soldier.  Soldiers are unlocked by the Planetary Mobilization tech, which comes before the Planetary Invasion tech, which unlocks transports.  You do not have the Planetary Mobilization tech, so you can't make a soldier to load onto the transport.  The transport is useless.

I'd be curious on how you got planetary invasion before Mobilizaiton in this case, are you running any particular mods by chance?

That should not be allowed. 

I have never to my knowledge played a game you could trade for a tech you did not have the prerequisite for.

When I first checked the save I thought it was early in the game.  I just checked again.  The save is at turn 23.  How did some AI get Planetary Invasion that early in order to trade it?  I think it's still too early for the event that gives the random tech, but maybe I'm wrong.  I did this tech from that event one time and also couldn't load a transport.  It took quite a while before I could figure out the "problem".  I don't think it was this early in the game.  Maybe I'm not remembering correctly how early in the game it was.

It could still happen this early by some method I'm not familiar with, or it could be the result of using a console command to directly get the tech without being aware of how much trouble it would cause.

I think we need a response from galactic-terraformers to get the real reason the Planetary Invastion tech was available on turn 23.

Unfortunately, it's been some time, I cannot recollect how I managed to get it, and there's no earlier save that I can check. This also calls for a couple of useful features: (1) saves keep a full log that can aid with tracing issues; and (2) saves were numbered by turn instead of just autosave and previous autosave, it would be possible to go back and dig out the older saves.

If you see the report tab, it does say that planetary invasion is the most value tech that I have bought, but that does not make sense because I have not encountered any major civilization from whom I can trade it. So that's also a bug in the reporting.

Perhaps game devs can say what possibilities are counted to have a tech go into the "most valuable tech bought" report, and we can narrow down the possibilities from there. 

In any case, this could be "fixed" by one of the following: (1) enable solders in both planetary mobilization or planetary invasion; (2) more generally, not allow any events, trading, etc. to give a tech that is not usable because some preceding tech has so far not been researched. The latter can lead to some interesting gameplay changes.

Btw this has happened to me a couple of times before, and back then I thought I must be doing something dumb. This is not all that rare.

It's not out of the realm of possibilities. Many companies have access to technologies they don't understand because another company laid the ground work to research for it.  

It's also generally doesn't come up in games that support tech trading that you need access to all of the techs leading up to it. 

Different is from Civ bonus or as this now appears to come from an event.
One Civ bonus is Starfaring that gives two techs in the slip stream travel and discovery.