Discourse + DigitalOcean + Mailjet setup

In round numbers, you have 1G RAM and 2G swap, and from the log we see ember ran out of memory. You have 16G free disk space - I’d suggest adding a 2G swapfile and trying again.

Edit: crossed in post. Indeed, a temporary upgrade to more RAM is another approach. Be sure to navigate the upgrade carefully so you can come back to the cheaper configuration - usually, that means keeping disk size the same.

Edit: there are a couple of kernel settings you should set to reduce consumption of virtual memory. See
MKJ’s Opinionated Discourse Deployment Configuration