[SOLVED] Server 2019 - Cannot install roles/features. 0x800f0831 & 0x80070057(?)

[TheOneGoofAli]

It's been a while since I installed any roles to Server 2019, but when I tried to install one, this time, 0x800f0831 shows up, sometimes 0x80070057 instead. DISM doesn't get to finish either, complaining about missing files even if I point it to an appropriate Server 2019 .wim... neither does SFC.

 

[TheOneGoofAli]

Fixed by manually pointing SFC/DISM to the missing packages, as well as manually tossing in some missing ones that the former did not copy over.