You need at least 1G of swap space - do you have it?
free
will tell you.
Edit: I recommend you read
MKJ’s Opinionated Discourse Deployment Configuration
too. You have this warning which is not good on a small memory system
See the Kernel Configuration section - you need these effects, although whether it’s exactly these commands may depend on which distribution you are running:
Edit: you can check these two important settings on your running server like this
# cat /proc/sys/vm/overcommit_memory
1
# cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]