Problem to create RDM Disk (Option greyed out)

Hi,

I have a problem to create a VM with a RDM disk. Option is greyed out. I present LUNs to ESX

Configuration :                                                                               

Local Disks 270 Gb

VirtualSAN 80Gb

I present to ESX two LUNs :

80 Gb (format as datastore)

10Gb (for RDM)