By default LibreELEC does not have a swapfile.
You can enable a swapfile by:
cp /etc/swap.conf /storage/.config
then edit it and change
SWAP_ENABLED="no" to SWAP_ENABLED="yes".
SWAPFILESIZE="128" to your required swapfile size - normally 4 x RAM but RAM should be enough for a normal LE system.
When you reboot, a swapfile the size of SWAPFILESIZE is generated in /storage/.config/swapfile.