DIAGNOSIS The printer itself has limited RAM to process complex print jobs before sending them to the physical print engine. A large, high-resolution PDF or a document with lots of images can exceed that buffer.
FIX Don't hit 'OK' to clear the error on the printer—that will cancel the job. Go back to your computer. In your print dialog box, look for 'Properties' or 'Printer Properties'. Find the 'Paper/Quality' tab. Change the 'Print Quality' from 'Best' or 'Maximum DPI' down to 'Normal' or 'Draft'. This reduces the amount of data the printer has to process. Also, make sure you're printing the 'Actual Size' and not 'Fit to Page', which can add processing overhead.
IF THAT DIDN'T WORK Break the job up. If it's a PDF, try printing a subset of pages (like 1-5, then 6-10) instead of the whole document at once. Alternatively, on your computer, use software to reduce the PDF's resolution or compress the images before sending it to print.
HEADS UP This error is about the printer's processing memory, not its storage or your computer's memory. Upgrading your computer's RAM won't help. This is a hardware limitation of the printer, so reducing the complexity of the job is the only real fix.