Im having some issues of my own with the Flak truck. It seems recent changes have broken my mod and the OKW are unplayable without the truck working.
I don't suppose anyone has the old code for how the truck used to work? (deploy and it would shoot ground and air - no upgrade required other than one truck deployed).
In ebps\races\west_german\buildings\base_building\infantry_support_mp,
In ebps\construction_ext\on_construction_actions:
remove apply_modifier_action\3. enable_weapon_modifier (value -1 disables the gun).
Als remove the two remove_crew_actions from the on_construction_actions.
In ebps\action_apply\actions remove 6. and 7. requirement actions. These activate the gun and crew when the upgrade is bought.
If you want it to fire on planes by default:
Set in weapon\west_german\small_arms\tank_gun\base_building_flak_20mm_flak38, behaviour\aa_weapon to true.
p.s. Next time please make a new topic for a new question, that makes it easier to find and differentiate topics.