I think your hardware is unsupported. Did you check the VMware HCL?
What is the processor model.
verify vmware hardware compatibility guide:
Most likely your hardware is not supported.
VMware Compatibility Guide - System Search
Enter your model here
Please consider marking this answer "correct" or "helpful" if you think your question have been answered correctly.
As others have already mentioned here, a Dell Optiplex 755 is not a suitable hardware platform for ESXi. However, it is possible to install ESXi on these machines, but you probably need a different CPU.
The error message looks like a CPU is installed that is no longer supported or accepted by ESXi 6.5. See the release notes for a list of unsupported CPUs for ESXi 6.5:
VMware vSphere 6.5 Release Notes
And you may find more information in the Dell communities:
Optiplex 755 and ESXi 6.5 Core 2 CPU Compatibility - Dell Community
Check this , it may helpful
Optiplex 755 and ESXi 6.5 Core 2 CPU Compatibility - Dell Community