Memory (RAM) and storage (hard disk / SSD) are not related to one another.
Check Memory Mac
That Mac has plenty of available storage. If you are running low on memory Activity Monitor can be used to identify the memory-intensive processes causing that warning.
To learn how to use Activity Monitor please read the Activity Monitor User Guide. For memory usage, refer to View memory usage in Activity Monitor on Mac.
Immediately test your Mac's RAM upgrade. Without thorough testing, faults in RAM can go unnoticed until problems start occurring in your system. Make it a priority to run a memory test on the. How to test for bad or faulty RAM on a Mac: How to replace the RAM in your Mac If repeated tests indicate that something is wrong with your Mac's RAM, it's time to do something about the problem.
Test Memory Mac Pro
Test Memory For Macbook Air
Once you determine the memory-intensive process or processes, a solution can be provided. WIthout that information it is premature to draw any conclusions, but the number one explanation for that warning is having inadvertently installed adware. To learn how to recognize adware so that you do not install it, please read How to install adware - Apple Community.