Warning that system has run out of application memory

The biggest thing to understand here is the difference between memory and storage. Memory is what your computer uses in the background to do everything while you're using it. If it has been a while, I would suggest restarting your computer to allow it to clear out any memory that might be getting used by lingering applications that are running, or even following what the system is telling you in the warning message and close out any applications that the computer is currently running that you aren't using. You can always open the Activity Monitor to see what applications are using what and how much of your RAM (Memory) is being used for what.

Storage is what your computer uses for storing everything on your computer. When you say you're not low on memory, the only way that you're going to be able to tell this is by using an application like Activity Monitor and see what is using the memory and see how much is going where. With storage (Hard Drive) you can look and see what is being used up by going to System Settings > General > Storage this will tell you what is using up the storage of your computer and help you find what you can remove to get back some of your storage space. Often times, people get these confused and when they hear or see memory they associate it with storage. Your computer can still have ample storage but the applications running could be using up all the memory and no matter how much storage you have, if your memory gets used up, you're going to see that message letting you know that it has been. Memory can change in an instant, if it's used up and you close an application, when the application closes it releases the memory that it is using and you get that memory back for another application to run. Restarting the computer can help as well as it will clear out the memory when the computer reboots and non essential things that might be running at the time using up memory may not run again right away when the computer reboots allowing that memory to be used by other applications again.

You will need to provide the free EtreCheck report to see what is installed that is taking up so much memory. There is something else running on startup consuming RAM, so there is not enough left for your Applications.

Using EtreCheck - Apple Community