Use ${env.ProgramFiles(x86)}
. The variable is case-sensitive in a way that is inconsistent with env.PROGRAMFILES.
[echo] env.ProgramFiles(x86) == C:\Program Files (x86)
[echo] env.PROGRAMFILES(x86) == ${env.PROGRAMFILES(x86)}
[echo] env.ProgramFiles == ${env.ProgramFiles}
[echo] env.PROGRAMFILES == C:\Program Files