There is a join which has no any sense because there are no any related ends of walls in scrap alien_weaponry_1p connected to scrap marniosa_upstream_4-upstream4p

If you comment next join and correct the overlapping it should work.

#join alien_weaponry-1p@alien_weaponry marniosa_upstream_4-upstream4p@marniosa_upstream_4

Anyway you may join scrap marniosa_upstream_4-upstream4p to pit in scrap alien_weaponry_1p manually as Bruce wrote.

Joining of scraps is not fully automated process, there are situations where it will not work.

Martin