Only yesterday I managed to revive a desktop computer that I had forgotten about. 
When I found it had Manjaro Linux, I ran the “Software Update” program. “Software Update” appeared to have worked (although I can’t be sure). However, when “Software Update” had finished, I got 24 error messages. The first, the 12th and the last are shown below:
Error: audit: signature from “Levente Polyak (anthraxx) firstname.lastname@example.org” is unknown trust
Error: jre8-openjdk: signature from “Levente Polyak (anthraxx) email@example.com” is unknown trust
Error: xed: signature from “Eli Schwartz firstname.lastname@example.org” is unknown trust
… the full set of messages are at https://paste.ee/p/B3P6E
Since that upgrade, I have not been able to install any other package. Each time I try, unsuccessfully, to install any single software package, I get the same messages. The most recent package. which I unsuccessfully attempted to install, is the Bless Hex Editor.
Can someone please why I get these messages and suggest a remedy?
 It has 3 hard disks and 8Gigs of RAM. That amount of RAM surprised me. I did not think I owned any desktops with more than 4G of RAM. See footnote 
 My home directory tells me I have used it as recently as October 2018, but I have no recollection. if I did use it that recently and have forgotten, it may be a consequence of the diffuse axonal brain damage which I suffered when a 4WD ran into me eight and a half years ago.