can't use faith to buy scientists or merchants

I've chosen Rationalism and Commerce but cannot use faith to buy either a GS or GM despite having >1k saved. The option just isn't there for any of my cities. All have my religion and there are no noncombat units in the way. The faith purchase option is on "Remind me later". Tried closing out the game and starting again. Is there any explanation? Help! My faith keeps going to more and more Prophets, which I no longer need.

Era has to be industrial or later (if prophets are still auto-spawning, that is pre-industrial).

Policy trees that grant great people have to be completely filled before that perk comes online.

The thread is tagged G&K, so the second part of GhostSalsa's response is not applicable (just have to open the trees in G&K). But, the first part is spot on -- if GPs are still auto-spawning, you aren't in the Industrial Era yet, so no faith-purchasing of Great Persons.

Thank you! Not sure how I missed learning that fact in my hundreds of hours.