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:

The error:

# ls
 Could not load program ls: Cannot run a 64-bit program until the 64-bit
 environment has been configured. See the system administrator.

The correction:

# /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