Upgrade your current version of ESXi 6.7.
See https://knowledgegainedandshared.com/2017/05/13/easy-esxi-patch-updates/. You could also see the No Space left on the device error, I did.
Reboot the server and follow the steps posted below, except using the ESXi 7 ISO.
Verify any Unsupported Device Warnings are an issue.
To determine what the hardware component do the following after SSHing into the host.
[root@esxi1:~] lspci -p | grep 1000
0000:02:00.0 1000:0064 15d9:083c 11/ / A V
Then take the Vendor Device Id (the second section of numbers) and grep for it.
[root@esxi1:~] lspci | grep 0000:02:00.0
0000:02:00.0 Serial Attached SCSI controller: Broadcom / LSI SAS2116 PCI-Express Fusion-MPT SAS-2 [Meteor]
In my case its on old onboard SAS controller that I do not utilize. If I had disabled the controller in the BIOS, I would not have had the warning. Installing ESXi 7.0 went without a hitch after ignoring the error.