As written in the changelog, you need to have 15 donations.
I know this seems quite a lot - the problem being that it was previously artificially limited to 8 but I balanced this number now on a complete playthrough where I think 15 is reasonable. Because of the previous limit, there is a big gap now that is not really intended. But if the general feedback is that this is too grindy, I can reduce it in a future update.
Either way, it should be pretty easy to catch up if you spam an interaction with Clara, donate, sleep, repeat . Just takes a bit of clickety-click.