There is an incompatible datastore block size. For example, you are trying to move a virtual disk bigger than 256 GB into a 1MB block size datastore.
There is not enough free space
Resolution
To resolve this issue, use one of these options:
In the Select Datastore step of the wizard, choose another datastore that meets all of the requirements.
By default, Storage vMotion assumes that all of the virtual machines are going to the same destination. To resolve this, click Advanced in the Select Datastore step of the wizard and choose the individual destination for the configuration file and for every disk.
When cloning a virtual machine, ensure that the destination datastore meets the block size limitations for all the virtual disks