Assume for the moment that the error message is actually correct. Check all the existing DHCP Lease information to see if that IP address, MAC, or hostname was already assigned. DHCP can hold inactive lease information for a while.
Also check Diagnostics->ARP Table, to see if that IP address is already listed to a different MAC address. It's possible that the address was used in a local static assignment already, not involving DHCP. You should do this shortly after trying to assign the static mapping as ARP entries will expire from the cache. Note: if DHCP relaying is involved you'll need to check this on a box attached to the subnet in question, using "arp -a" (works on *nix and Windows).
Originally I thought that it could have been you were trying to make permanent the IP address it got dynamically from the DHCP Pool, but that yields a very specific error message to that effect. At least in version 2.4.3.
