I found in Archer C6 a probably all new models you implemented a check - you can not set different IP-MAC combination in DHCP Static list and ARP Bind list.
Idea is good unfortunatelly this cause a lot of troubles:
1. if you need change (switch) for example 2 devices, it is not possible, you can not change DHCP Static because of its ARP Record, you can not change ARP records because you have the device in DHCP Static record.
You need do it like this: delete DHCP Static record, delete ARP Static record, power off devices so they won't be in ARP devices list, then add DHCP static and ARP static records, then you can power on the devices
As you can see, the DHCP <->ARP check is not helpful
2. you also can NOT change Description in DHCP or in ARP list, because you can NOT edit either record, because the record is already in the other list.
3. Also all this is not correctly explained, you see only error "The to-be-added address already exists." and you have no clue that this is about DHCP static <-> ARP static crosscheck, it just looks like it is a bug (This crosscheck was not present in previous TP-Link models).
A. quickfix: remove DHCP static <-> ARP static crosscheck
B. if you want this check to be present, put DHCP and ARP static list on one page or in one table, or something like this
pls be sure this will be reported and fixed in next firmware