An explanation for what the problem is can be found in THIS
The solution will involve editing the ships.tbl file.
Entries for the Aurora, Aurora MK1 and PSI Aurora all have the following line in their tables:
$Default PBanks: ( "40mm Pulse" "" )
The game is concerned that the model has 2 primary weapon banks and the table file only has 1, since the second is left blank in the tables. The solution is to add a valid weapon entry into between the 2 quotation marks. You can use anything from the Allowed Banks list, or just add "<NONE>", which is itself a valid weapon from the weapons table. Depending on which campaign you are trying to play, you might be able to play it without making these changes, so long as none of these Aurora variants are in the mission. Something like Raider Wars should only use the standard Aurora, while the Minbari War campaigns would use the MK1. Either way, I'd suggest making the fix for all of the three variants to be on the safe side. Remember to re-save the ships.tbl file after you make the changes. It should be noted that the Oct. 7th build restores the message warning about this kind of thing, which had been removed in the late summer and is still missing from the 21.4 build.
I hope this fixes things for you.