did you already request one for that ESX server ?  you can try to modify to use a different UUID I think someone posted the instructions here on the forum
Hi all,  1. I the UnityVSA OVA (4.3.0.1518191253) downloaded & deployed it into my vSphere cluster successfully. 2. When I attempt to get license the file during SETUP by the link "Get License Online", I get the error:     "The UUID entered already exists. Validate your System UUID or contact our World Wide License Support team below for additional assistance. Contact EMCs live support team by calling - 800-782-4362, option 4, option 4 (24 X 7)" 3. When I attempt to get license the file directly at [URL] I get the same error. 4. Support at 800-782-4362 option 4 says they cannot support me without a serial number or site ID - I have not encountered either for this simulation product  Please advise me on how to get the license for the Unity VSA (downloaded at: Dell EMC UnityVSA Download | Dell EMC
UnityVSA - Unable to get license