I have enabled homebrew in my characters sheet but I still am unable to add homebrew spells as there is no button to do so as there is in inventory.

What am I missing?

You would add them through the normal spell management screen in the character builder, not on the character sheet.

It is not an option. Below is what it shows. 

FILTER BY SOURCE CATEGORY
Core Rules |  Expanded Rules | 2014 Core Rules

I also ensured that the "Homebrew" option is checked again. It is. 

https://www.dndbeyond.com/characters/101691835/8xSX89

You need to add the homebrew spell(s) you want to your collection for them to appear in the character builder.