Over the last couple of weeks there have been several reports coming in from customers that the local SSD device found in newer Apple Mac Pro 6,1 were no longer being detected by ESXi. Starting with ESXi 5.5 Patch03 and ESXi 6.0, the Apple Mac Pro 6,1 was officially supported but it looks like the latest versions of the Mac Pro 6,1 that are being shipped contain a slightly different local SSD device which is not recognized by ESXi.
This was not the first time that this has happened, when the 2014 Mac Mini were first released, they too had a similar issue in which a custom VIB was required to get the internal device to get recognized by ESXi. There is an internal bug (PR 1487494) that is currently tracking the issue and if you are also experiencing this problem, please file an SR and have the GSS Engineer attach your case to this bug.
In the meantime, there is an unofficial workaround which was discovered by one of my readers (Mr. Spock) that by installing the community SATA-XACHI VIB over on Andreas Peetz VIB Depot site, both ESXi 5.5 and ESXi 6.0 will then recognize the local SSD Device. You will need to either use VMware Image Builder or Andreas ESXi Customizer tool to create a custom image if you decide to install ESXi directly on the local SSD device. I personally would recommend installing ESXi on USB device, this would allow you to install the VIB as a post-installation and not requiring a custom ESXi image.