In ESXi 6.5 Physical NIC where is auto negotiate option in GUI as i cannot see this in edit adapter option?
On 10GbE NICs autonegotiation is not supported.
You are using VMXNET NIC (nested ESXi, 10GbE emulated speed)
Dont worry, its not a bug it should looks like that.
I happened to notice the same issue in ESXi 6.0 update3
I couldn't see the Auto-negotiate option too and have intel x540-at2 network interface on ESXi host.
Putty to the ESXi host and enable auto negotiation on the interface from command line
1. # esxcfg-nics -a <vmnic2>
2. Login into vSphere Client and you should see the autonegotiate appear.
Worked for me with ESXi 6.0 update3