Server = http://viridian-project.de/~sky/paktahn/repo/x86_64

If you’ve already installed the 32-bit version as I had, this is how I fixed it.

1) edit /etc/pacman.conf and comment out the [paktahn] repo.

2) sudo pacman -R paktahn && sudo pacman -Sc
(Answer yes to all questions)

3) edit /etc/pacman.conf and uncomment the [paktahn] repo. Change the server line to :-
Server = http://viridian-project.de/~sky/paktahn/repo/x86_64

4) sudo pacman -Sy paktahn

However, it seems broken for me.

johncc@liberator:~$ paktahn
unhandled SIMPLE-ERROR in thread #:
Error opening shared object “libreadline.so.6″:
libreadline.so.6: wrong ELF class: ELFCLASS64.

0: (SB-DEBUG::MAP-BACKTRACE #)[:EXTERNAL]
1: (BACKTRACE 128 #)
2: (SB-DEBUG::DEBUGGER-DISABLED-HOOK
#
#)
3: (INVOKE-DEBUGGER #)
4: (ERROR “Error opening ~:[runtime~;shared object ~:*~S~]:~% ~A.”)[:EXTERNAL]
5: (SB-SYS:DLOPEN-OR-LOSE
#S(SB-ALIEN::SHARED-OBJECT
:PATHNAME #P”libreadline.so.6″
:NAMESTRING “libreadline.so.6″
:HANDLE NIL
:DONT-SAVE NIL))
6: (SB-ALIEN::TRY-REOPEN-SHARED-OBJECT
#S(SB-ALIEN::SHARED-OBJECT
:PATHNAME #P”libreadline.so.6″
:NAMESTRING “libreadline.so.6″
:HANDLE NIL
:DONT-SAVE NIL))
7: (SB-SYS:REOPEN-SHARED-OBJECTS)
8: (SB-IMPL::FOREIGN-REINIT)
9: (SB-IMPL::REINIT)
10: ((LABELS SB-IMPL::RESTART-LISP))

unhandled condition in –disable-debugger mode, quitting