The reason for this is most likely that there's not enough disk space left on the datastore to create the swap (.vswp) file which - once the VM is powered on - is created with the size of the VM's provisioned memory minus the reserved memory. There are actually options for you now. Either convert the VM to reduce its virtual disk size, or set the reserved memory (VM Settings - Options tab) for the VM to the amount of the provisioned memory. The latter however requires that the host has sufficient physical memory to run all VMs.
Remember that with only 2GB free disk space it may be risky to run VM's, especially if they have thin provisioned virtual disks, or snapshots!
André