When you installed Ghost via Digital Ocean, and you want to upgrade - you might see this error message.
You are recommended to have at least 150 MB of memory available for smooth operation. It looks like you have ~35 MB available.
This can happen because you are probably using $5 droplet with limited memory. And in my case, I started to upgrade a lot of images-screenshots, which seems like consumed a lot of memory.
Thankfully, there is a solution!
You need to run ghost upgrade without memory check.
Run the following command to update ghost in Terminal.
ghost update --no-mem-check