For our environment, our server was deployed 5+ years ago with an enterprise license which does not appear to include our Serial Number. In order to find ours, if you log into the CLI with the ampadmin account and use option 11, use the following command:
dmidecode - Display serial number of the server
It will provide an output similar to:
System Information
Manufacturer: HP
Product Name: ProLiant DL360p Gen8
Version: Not Specified
Serial Number: XXXXXXXXX
UUID: XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXX
Wake-up Type: Power Switch
SKU Number: XXXXXX-XXX
Family: ProLiant
Just make sure you are logging the CLI session as it will generate a long string of data and you will likely need to go back to where you issued the command, this is the second thing it provides. I had to go back to the log in order to scroll back enough to find it.