Ran out of application memory

This is a question that comes up fairly often. The error message logically sounds like the problem is that your Mac doesn't have enough memory (RAM), but I've yet to hear of an instance where this was actually the problem.

In every case the problem turned out to be that the user's hard drive was too full and it was at the point where either a large amount of data had to be offloaded from the drive and, in the case of a rotating disk hard drive, then erased (and, even better, the hard drive then defragmented), or the drive needed to be replaced with a new, much bigger, drive. (DO NOT try and defragment or erase an SSD!)

You should note that when a rotating disk hard drive is around 80% full (as yours is), it's going to start becoming problematic:

Macintosh Routine Maintenance
Item #5 and Note #1

Or, if you have an SSD, they are done when they are around 70% full:

"In practice, an SSD’s performance begins to decline after it reaches about 50% full."
https://www.seagate.com/tech-insights/ssd-over-provisioning-benefits-master-ti/

"The rule of thumb to keep SSDs at top speeds is to never completely fill them up. To avoid performance issues, you should never use more than 70% of its total capacity.
...
"When you’re getting close to the 70% threshold, you should consider upgrading your computer’s SSD with a larger drive."
Why solid-state drive (SSD) performance slows down as it becomes full - Pureinfotech

Reboot said:
"Also as far as someone mentioned about needing free contiguous space, the modern OS X also automatically defragments hard drives in the background."

The Macintosh OS has been automatically defragmenting the hard drive for many years. Unfortunately, in this case, that is of no help. Because while the Mac OS does indeed defragment applications and the like, it does not do whole disk defragmentation. See:

Item #5 and Note #1

unblocktheplanet said:

Big worry for me is, this is a 2015 Mac...

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Macintoshes have been excellent computers for decades. I just about never hear of anyone whose Mac has failed and they needed to purchase a new one. Almost always Mac users' Macs last until the day comes that the user simply is desirous to have a new one.

I can't tell you how many Mac users I've met who have a closet full of old Macs, all of which still work perfectly. They all simply got to an age where the user decided to retire them, but couldn't bring themselves to throw the old one out because it still worked.

pm-r said:

I dare say that there would be a lot more of them still in use only if the software they need to run modern stuff could be updated somehow. Not everyone needs the speediest machines going for what the average person uses their computer for.

Click to expand...

It's not simply that modern software applications would have to be written to be compatible with that old hardware. It would also have to be written to be slim enough (for want of a better word) so that it didn't tax the processing power of the old machine.

I have a 15 year old iMac running in my office. I keep it in service to run a couple of legacy applications that were never updated for later machines, and nothing modern is as good. (e.g. OmniPage Pro OCR.) The machine will run some software that came out well after that iMac was a terribly outdated personal computer. But newer software runs a bit slowly on such an old computer, and it's annoying.

The thing is, many of us don't want to use our old computers forever. We want all of the benefits of having a new, really fast, cutting edge, computer. At least for all around general use.