The specified IP address range is too small.
Currently, the user class option field permits only one ASCII text string to be used for identifying clients. This means each client computer can only be identified as a member of a single user class by the DHCP server. If you need to, you can use additional user classes and make new hybrids from your other user classes.
Check to see that the class name is correctly configured at either the server, scope, client, or class level.