Hey elen179,
We're glad you thought to ask Apple Support Communities about the alert that has appeared saying you're out of application memory. To start, how much free disk space do you have available? To find out select the Apple menu > About This Mac > Storage. You'll want to ensure you have plenty of available disk space to help with this: Optimize storage space on your Mac
You'll also want to use Activity Monitor to see what app or process is using the most memory: View memory usage in Activity Monitor on Mac
Let us know what you find.
Take care.