Okay I see.
The issue is caused by a name change in the weapon declaration for the Bf 109G-6.
I have no idea what the idea nor the reason behind changing names for existing loadouts is, but as a matter of fact, it will break existing missions and campaigns.
The cause of this is that BAT includes the
Bf-109 Ultimate Pack v4, with enhanced loadouts, but unfortunately also with some identical loadouts under different names.
Here is a list of Bf 109G-6 loadouts for example:
Stock 4.12.2 Bf 109 Pack
default default
R1-SC250 R1-SC250
R1-AB250
R1-SC500 R1-SC500
R1-AB500
R2-SC50 R2-SC50
R2-AB23
2xWfrGr21 R2-WfrGr21
R3-DROPTANK R3-DROPTANK
R5-MK108 R4-2XMK108
R6-MG151-20 R6-2XMG151-20
2xWfrGr21-R3 R2R3-TANKWfrGr21
R3R6-MG151-20 R3R6-MG151-20
R3R4-2XMK108
U3-MK108 U3-MK108
U4R2-MK108WfrGr21
U4R3-MK108 U4R3-TANK1XMK108
U4R4-3XMK108
U3R6-MG151-20 U4R6-MK1082XMG151-20
U4R3R4-3XMK108
U4R3R6-MK1082XMG151-20
U3-NOMG131
U3R3-TANK-NOMG131
R2-RECON
R2-RECON-DROPTANK
R3-RECON-2XDROPTANK
none none
In your particular case, the mission wants to load a Bf 109G-6 with "U3R6-MG151-20" loadout, but that doesn't exist in the
Bf-109 Ultimate Pack v4 (and therefore also not in BAT), it would have to be renamed to "U4R6-MK1082XMG151-20" in the mission file.
Best regards - Mike