Hi On the P300 you cannot set controller, as there is no controller supported. P300 is meant for bridge connection. No vSZ/SZ/SCG or ZD support on it.
the other AP you should be able to set the controller ip. Usually customers use CLI with set director or with the set scg ip command. If you cannot the AP might be under control of a controller already, you can factory reset it and then try again.
I also noticed this issue when configuring the R500 with an IP for the controller but with a different set of choices (see below). I enabled the first 2 options and then the "Set Controller Address" was now active. Seems weird but worked for me in this environment as I don't have any DHCP/DNS running.