Why can’t I assign an IP address using a static entry in the ARP cache?

This method can only be used if the Web-IO does not currently have an IP address (see also How do you assign a new IP address to a Web-IO already having an unknown address?).
Some older Windows versions have a defective version of the program arp.exe, which is needed to edit the system-internal ARP cache. Here you can only make a new static entry if at least one other entry already exists. In this case, one remedy is to send a PING request to another TCP/IP station in the network, since this will force a dynamic ARP entry. The desired static entry can then be made immediately following.

Additional information: