Came across this silly error while trying to enter DHCP reservations into Server 2012 R2. The issue is not related to the IP being outside the DHCP address pool.
You get the following (un)helpful warnings:
“The Unique Identifier you have entered may not be correct. Do you
want to use this identifier anyway”
“The DHCP server received a message from a client that is not valid”
Which tells you nothing helpful or informative. After adjusting the Address Pool, it because clear that was not the issue. Using a random MAC address generator, the generated MAC worked without issue. Ironically it was lowercase. I converted my failed MAC address to lowercase and it worked without issue, even outside the Address Pool.
Ideally this saves others a few minutes in their day.