Hello Andrei Demkin,
Sorry for the inconvenience you're experiencing.
The "Select" field you're referring to is for popups created using the SP Page Builder Popup Builder. If the dropdown is empty, it means you haven't created a popup yet. Once you create a popup in SP Page Builder, it will automatically appear in the list, allowing you to select it and link it to the button's open/close action.
We appreciate your feedback and will be sure to add this clarification to our documentation.
If you need assistance with setting up the popup, please let me know. If this doesn't solve the issue, please reach out and I'll look into it more deeply.
Best regards