Q: Can't use mac at all because of application memory error...

Computers have what's known as disk or SSD storage — that is the storage where your data and pictures and files and applications all reside, and particularly when you're not using them — and also have what is called random access memory — that's where your data or pictures or applications (briefly!) reside, and only when you are actively using them. 

Disk or SSD storage is big and relatively slow, and random access memory storage is much smaller but vastly faster.   OS X will move the data from disk or SSD into random access memory just for as long as that's necessary to work on what you've asked OS X to do, and any changes and any updates will be written from random access memory back out to disk or SSD storage.   When the application is quit, it's removed from random access memory.

The error you're reporting is likely insufficient random access memory.  Deleting files off the disk or SSD won't help with that.

One common source of this is either too little random access memory for all of what you've asked OS X to do —  > About This Mac will tell you how much "memory" you have — this is random access memory — and how much storage — this is disk or SSD storage — you have available.    It's also possible that some application is not releasing random access memory for some reason such as a corruption or a programming bug.

Activity Manager is an application in the Utilities folder within your Applications folder.   Using Finder, open Applications folder, the open Utilities, then launch Activity Manager.   Depending on exactly which version of OS X you're using, Activity Monitor will show you a graphic display of how much CPU, memory, energy, disk and network activity is occurring.

There's a long thread discussing this topic that might be worth a review — though it's quite possible your situation will have a very different trigger.

If you want to discuss this here, can you confirm which version of OS X you are using — see the  > About This Mac — This is posted in the 10.9 Mavericks forum — and then launch Activity Monitor, and try to identify which application is using random access memory.

Try adding more RAM. The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing

The 2 causes I know about are.

a) your boot disk is full and new /var/vm/swapfile(s) cannot be created.

b) too much virtual memory is being used, such that the OS X kernel has run out of page table entries to manage all the virtual memory the current applications and processes want to use.

In either case, it can be a single application or process that is going crazy, or a set of apps and processes that combine to uses excessive virtual memory.

Not going to add more RAM to a 2014 MacBook Air.

In your Dock, click Launchpad (rocket on the left end), and on the first screen, there will be a collection of icons called Other. Click once. Click once again on Activity Monitor.

Select the Memory tab, and at the bottom left, a normally operating 2014 MacBook Air with El Capitan (OS X 10.11.1) will have green memory pressure. If yours shows red, there is an issue with something eating your available RAM, which probably is 4GB. While still in the Memory tab, locate the Memory heading over the list of running processes. The arrow on it should be pointed downwards (click it if it isn't), to show descending memory consumption by processes. The Kernel on my 2014 MacBook Air shows 709 MB, and that is normal. I also have a very small band of green memory pressure. Adjacent to the memory pressure box, how much swap does it show that you have used?

Switch from the large memory tab over to the CPU tab. Again, if the arrow on % CPU is not pointing downward, click once to show descending CPU usage. The Activity Monitor will appear, as well as the Kernel task. So will a Safari session if you are reading this. Anything else hogging the top slot?

Quit Activity Monitor. Back in Launchpad : Other, click on Console. By default, All Messages will be selected, and in the lower right corner, Now should be selected. Most recent activity is at the bottom. In the Console Search field, type in Error, and see if there is anything that stands out in your messages. Then repeat this again for Failed, and Warning. If you see heavy logging of the same activity, it may be what is consuming your memory. Quit Console when you are done.

A lot of people have said on similar threads to just buy more RAM but that seemed like a bandaid fix, and I only bought my computer a few months ago (and have a ton of free memory). None of my business, but I could see Apple going along with this because they don't get enough of our money in the first place...

Since Apple is building more and more computers with a fixed amount of RAM, I find it highly unlikely that Apple wants you to buy more RAM.

Also this is a user-to-user forum, so if you want to tell Apple something, I suggest you use