You have a hardware problem which must be addressed.
The first thing to try is to remove and reinstall the memory. I'm not sure about this exact model, but the memory is most likely installed under a trap door on the bottom of the machine. The memory can be damaged by static electric discharge, and you must also be careful with the clips that hold it in place, as they are often made of plastic and can break easily if pushed too far.
To deal with static electricity, sit down, don't slide around, and always touch a metal part of the computer case before touching any components. There are various other precautions you can take, but those are the main ones.
You can search and find explanations for how to remove and replace memory if it doesn't look too obvious for you.
If, after removing and reinstalling the memory, you still have the memory parity error, then you have more work to do. First, if you have two memory modules, you should try removing one or the other and see if the computer works fine with only one. This will help you determine which module is defective.
Second, you may wish to purchase or otherwise acquire replacement memory. You might do this whether or not you were able to operate with only one module, since your computer will operate better if it has enough memory compared to having too little memory available.