Man the gun physically. This is normal behavior for Blazemod mounted guns, they’re changed to be manual-only. Find the seat where the actual cannon is installed, sit there, unwield whatever weapon you might be holding, fire.
Blaze seriously needs to put some fair warning in the description of Vehicle Additions Pack.
It’s actually my fault - I forgot to disable turret enable/disable and warnings for manual-only turrets. It should explicitly say “turrets are enabled, but no turret has autotargeting ability, they all need to be manned”