I had this same issue (I created a separate thread as for some reason couldn’t find this one when searching)
The preview link worked for me, at least as far as re-entering the ElevenLabs API key. I’ve not seen the custom voices show up yet though I’m aware this can take some time.
Fortunately, I also success to register 11labs api key via preview.convai.com. The custom voice hasn’t been appeared in my character’s voice list yet but I can wait with hope. Many thanks to convai team’s consideration and response. I’ll post this result. Thank you.