Hello,
Could you try using the command opkg install open-plc-utils?
I'm not very familiar with this package, but the way you're trying to install it is to compile it locally. RUT240 simply does not have enough storage or processing power for that, but the way I suggested, you can download an already compiled package. I've successfully downloaded it on RUTX11.
Let me know if anything else is needed.
Best regards,
DaumantasG