It depends on the CPU version. For Intel Xeon E5-2620 v3 CPUs the highest ESXi version supported is 6.0 U3 and for the v4 version of the same CPU model you can also install all ESXi 6.5 versions up to Update 2:
VMware Compatibility Guide - System Search
You can check the CPU version in the vSphere Web Client if you select the ESXi Host and go to the summary tab or in the Host Client in the Host overview (start page) under CPU.
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Regards,
Sebastian
VCP6.5-DCV // VCP7-CMA // vSAN 2017 Specialist
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