You should not get this under normal conditions or at least I have never met this under normal AIX administration.
However, if you are doing some operations in maintenance mode, you could bump into this error.
But as much as difficult it sounds, it is quite simple to solve:
# ls Could not load program ls: Cannot run a 64-bit program until the 64-bit environment has been configured. See the system administrator.
# /etc/methods/cfg64 #
Now ls is working:
# ls .toc lvm.cfg devices.fcp.disk.ibm.mpio.rte rmSDDPCMandHA.out devices.sddpcm.71.rte rmSDDPCMandHA.sh failed.cfg