your system has run out of application memory

There are 2 reasons for the “Your system has run out of application memory” dialog box.

A. Your boot disk has very low free storage, and macOS cannot create page/swap files to offload virtual memory contents to disk. Depending on how much virtual memory is being called for, anything under 50-100GB of free storage may trigger the message.

Apple menu (upper left corner) -> About This Mac -> Storage (tab)

B. A process (or set of processes) has asked macOS for excessive amounts of virtual memory address space. In order to keep track of the virtual memory address space, the kernel creates virtual memory page tables. If there is a memory leak (process asks for a virtual address range, forgets to give it back, asks for another range, forgets again, wash, rinse, repeat), eventually there are so many page tables created there is no memory left for applications, and you get the “Your system has run out of applications memory”.

Look at

Applications -> Utilities -> Activity Monitor -> View (menu) -> All Processes -> Memory (tab)

you can see what processes are using lots of memory. Many of these processes will be background agents and daemons used to provide many of the macOS services, as well as your applications.

Also keep in mind that each web browser tab will be a separate process running its own Javascript. If you have lots of browser tabs open, or if one of the browser tabs running Javascript with a bug in it, it is possible these browser tabs will add up to a lot of virtual memory demands, but no individual tab will look all that big.

Use the Activity Monitor application and make sure to use View >> View All Processes 

What process or processes are using the Most CPU  and Memory.

Often times - the issue is often not an individual and a single Process  alone but a combination of Processes

Also, when the computer is using so much Memory - it can be a indicator of a Lack Of Empty Spaces on the Drive.

In which case, the CPU and Memory usage would become Much More because it is searching for Empty Space to save all the Changes the user is performing.

Also, depending on the number of  Open Tabs in your Browser, they can also affect Memory Usage.

Having this error while the applications are using so little memory can, in my view, mean only one thing: your system drive is FULL.

You should always have some 15-20% free on your system drive. A full system drive can put you at risk of serious data loss.

If the user is using the Application in your previous link we refer to often times as " BrickMyMac  " or CleanMyMac suggest you remove this software as per the Developers Specific Instructions

In the screenshot, there are three applications. It is possible one of the applications has frozen due to some error. Try to update these applications, it may solve the issue.

Sorry 😔 but looking more closely at the OP Screen Shot indicates the 3 Application ( Activity Monitor, Safari and Finder ) are presenting Running and do not require updating