Why is SWAP not available for my VPS?

SWAP is not applicable to OpenVZ plans.

It is possible to use it with KVM plans, though. If you need help with SWAP setup, submit a corresponding ticket to the support department.

Adding a 512M swap file

dd if=/dev/zero of=/swap bs=1024 count=512K
mkswap /swap
chmod 0600 /swap
swapon /swap
echo "/swap swap swap defaults 0 0" | tee -a /etc/fstab