Rebecca is a teacher who is struggling with keeping her students interest during distance learning. Leo says that dialogue is the most important part of teaching. That question and answer time, rather than just play another youtube video for them. They can do that.
Her computer is running out of "application memory," even after installing a new hard drive. She uses macOS High Sierra. She tried to update, but it won't let her. Leo says that her computer is too old to update. Also, you may be running low on memory because your hard drive is running out of space. So try and free up at least 2GB or more of hard drive space. Also cleaning up your desktop can help.
But Rebecca should be able to install macOS Catalina and he suspects that the technician who replaced her hard drive did it wrong. Wipe the drive and do a clean install to the OS, update it, then restore your data and everything should be OK.