Hello,
A pair of IP addresses will be taken from /30 subnets for each VPN endpoint, one for the end device, the other for VPN gateway. This limits available IPs to the ones, provided in the table below, missing numbers being either a network address or a broadcast address, not usable for static leases.
Now the pair 1,2 is used by the server, thus the error message. However, if you were to take any other pair, you should have no issues to configure a static lease.
Best regards,