In the documentation for pspp 1.62, on page 275 (in my copy), in section 17.21 for the SHOW command, it shows DECIMALS (plural) is an available option.
However, in Section 17.20 for the SET command, the option is DECIMAL (singular).
It’s probably a typo but it could be confusing to the user. Certainly, SHOW DECIMALS gives an error if you try to use it, while SHOW DECIMAL does not (and shows the setting). SHOW ALL prints the DECIMAL setting, too.
This came up while I was looking into the ENVIRONMENT option, of course.