By default, the software installation process copies the configuration to the new boot location during the install, as described in K13438: Controlling configuration import when performing software upgrade/installation (https://my.f5.com/manage/s/article/K13438). If that is the case and no changes have been made since that installation was performed, it may not be necessary to install configuration, and the "Install Configuration" option can be set to "No".
Whenever i see such situation in GUI you do not see install button, either try using a different browser, and if the issue still persist from another browser I always boot to the new partition using the TMSH mode using following command and the configuration get copied automatically into that new partition from the present working partition,
In TMSH mode run this command
reboot volume HD1.4
F5 box will reboot to the new partition where you install new OS and it will automatically copy all the files from the Old working partition HD1.3
cpcfg command is more useful in those scenarios when you don't want the source partition to be the present working partition but some other partition which you can't select using GUI.