Create Swap space in ubuntu 18.04 aws

 Your problem is related to the probable fact of having only 1gb of RAM and not having memory swap, so let's create and enable it to take some time off at the time of composer execution.
Folow this steps:
1) Creating 1gb memory swapfile:
$ sudo fallocate -l 1G /swapfile 
$ sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576 
This command created the /swapfile file with 1GB in size.
2) Configuring file to swap:
$ sudo chmod 600 /swapfile 
$ sudo mkswap /swapfile 
3) Enable swap:
$ sudo swapon /swapfile 
To see result you can run free -m or see on htop
Example: $ sudo free -m
              total       usada       livre    compart.  buff/cache  disponível Mem.:          7664        1052        3436          56        3175        6259 Swap:          1024           0        1024 
Example: $ sudo swapon --show
NAME      TYPE SIZE USED PRIO /swapfile file   1G   0B   -2