The message is about the RAM, not the disk space. How much memory does your computer have? (4 GB, 8 GB, 16 GB)
Most likely you are using a 32-bit operating system?
Mac or Windows? What version?
JPG or raw files?
What action were you trying to perform in Lightroom when you got this error message? Please be very specific.
Sorry about not replying sooner. But the problem has disappeared for whatever reason.