Hi @Konvicka Ondrej , Thanks for using Microsoft Q&A Platform.
As a first step you might have created a language resource with the custom question answering feature enabled in the Azure portal. Please try to verify this part.
You can find the key by going to the "Keys and Endpoint" section of your deployed language resource in the Azure portal. Then copy and paste the one of the keys into the "Language Resource Key" field.
I hope this helps.
Regards,
Vasavi
-Please kindly accept the answer and vote 'yes' if you feel helpful to support the community, thanks.