Tried changing all possible language to English (US) and only English (US). Still greyed out.U need to change the language to English (US) first and removed the secondary language, then the voice typing will not grey out as the Google support page note mentioned works for English (US) and certain languages. After enabling it, u can then change the English to whichever u like, add back the secondary language.
Getting the Assistant voice typing to work is a bit on-off for me, need to try a few times before it start listening and type out what I said..lol..
I changed the language in:
1) System settings
2) Gboard
3) Google Assistant Language
4) Voice within Google app settings
5) Search language within Google app settings

