A quick fix could be to edit the file 'equipment.pzeqp' located in the PzC-Data-Folder. Look for entry 118, this should be "SdKfz 7", and edit the release date from 01.03.1940 to 01.09.1939. After restarting the scenario the transport should be available.
The "Achtacht" towing problem actually results from the trait "heavypull" new to the 'equipment.pzeqp'-file for some heavier units.
On one hand that is a very good idea, on the other hand the 8,8 Flak with a movement of one is rather useless until March 1940. This is most unfortunate, for I always buy one at the beginning of the game.
I like Zappel's solution (I guess you could remove "heavypull" from the 8,8's entry alternatively, but editing the SdKfz 7 is even historically correct, because it was built from 1938 on), but I would be interested if the developers didn't notice the effect "heavypull" had on the early scenarios especially the DLC or if it was intentional (not a bug but a feature ), maybe because the 8,8 Flak was too powerful early in the game? Would be nice to get an official statement...
Here is the official statement: the issue with 88mm gun is an omission, it was not done on purpose and will be fixed in the next update. In the meantime, it is perfectly ok to use cheats or replace the equipment file with this version: