On a system with a lower amount of RAM you may find that ESXi allocates too much to run the vmkernel and system services not leaving sufficient memory for running VMs. In the below example, the host has 3 GB of memory. ESXi is showing about 2600 MB available for virtual machines. But an important number for virtual machines is the memory capacity shown on the Resource Allocation in the 2nd image below. This shows that my VMs have a total capacity of just over 1400 MB for VM memory overhead. A chart of memory overhead per vCPU / VM memory is shown below. VM memory overhead includes space for the VM frame buffer and virtualization data structures like shadow page tables. Once my running VMs have exhausted the 1400 MB of total capacity, I will not be able to start additional VMs even though ESXi may have plenty of free memory.