I believe the (U)ser (I)nterface being referenced is the little networking icon (two computer monitors/displays stacked
by each other) located just to the left of the clock on Mate's panel/bar. Are you using Mate desktop of something else?
Permission denied is given when a normal user attempts to edit a system file or anything restricted to root. You will need to either "su" to super user (root) or simply use 'sudo' (super user do) which will ask for your normal user's log-in pass word. I.e. following the text in section 7.3 of the handbook at https://docs.freebsd.org/en/books/handbook/network/ and using sudo the commands will then be like this -->
sudo ifconfig em0 inet 192.168.1.150/24
`
and
sudo sysrc ifconfig_em0="inet 192.168.1.150 netmask 255.255.255.0"
