I want to send bulk SMS on WhatsApp without creating broadcast list.
For that reason, I found pywhatsapp package in python but it requires WhatsApp client registration through yowsup-cli.
So I've run yowsup-cli registration -r sms -C 00 -p 000000000000 which resulted in the error below:

INFO:yowsup.common.http.warequest:{"status":"fail","reason":"old_version"}
status: fail reason: old_version

What did I do wrong and how can I resolve this?
The error you get clearly points out the nature of the challenge you are facing: it has to do with your version of yowsup-cli; it's an old version.
It means that your project requires a version of yowsup-cli higher than what you currently have so as to work effectively as require.
What you need to do so as to resolve it is: to update your yowsup-cli application to a more recent version.
How to send bulk sms on whatsapp