How do i free unused ip adresses?

Hello,
I have recently replaced a device and the new one has to have the same ip because sone things (e.g my own fitbit app) are a pain to reconfig just because of one device.
The problem is that if i try to give the ip of the - removed - old device, it won’t accept it (the plus sign is grayed out). How can i free the address to use it with the new device??

that should just work. it’s not checking against other ips in the network. probably just confusing design or the address isn’t valid somehow.