How to activate POSIX mode on KSH93

Question

I have a script that is running on Redhat 5.11 machine with a type of SHELL pdksh.

However, when I run the same script on Redhat 7.1 with the KSH93 shell it gives me the following error:

./script.ksh[212]: set: posix: bad option(s) Usage: set [-sabefhkmnprtuvxBCGH] [-A name] [-o[option]] [arg ...]

Here is the line that generates the error:

set -o posix