Hi there Mariamariami,
Thank you for posting to Apple Support Communities.
We understand you'd like to set up your Apple Watch using Family Setup. We'd like to help.
It's a article you've linked with Set up Apple Watch for a family member - Apple Support
What we want to note Step 7 in the article:
"If your carrier doesn't support Family Setup, or you'd like to use a different carrier, select Not Now.* You can choose an alternative carrier from this list of supported carriers and contact them directly to set up your cellular plan.
Next, choose whether to share your current Wi-Fi network with the Apple Watch.
* Some corporate cellular plans don't support Family Setup."
You need to be sure that the carrier is a supported carrier and that you have selected "Not Now" during the initial Family Setup.
If you're still having an issue and the carrier is supported, we recommend contacting Apple Support for additional guidance: Get Support
Take care.