How do i block youtube

I was able to block youtube before from all devices by entering a lsit of website addresses in the Essential security tab - websites

Blocked Websites

Add websites that you always want parental controls to block access to

I removed the list for awhile and now wish to reblock it.

however, i cant add a single website to the list - everytime i clock save changes its gives the error message
Sorry the changes you made were not saved, please try again

I supose this is to force me to pay for the Advance security?

Any help or pointers greatly appriciated.