Hi There!
Thank you for your query.
Are you talking about the intro text, please?
You have to add intro text with default Joomla Editor. Write your intro text and insert "read more" from Joomla Editor. To create an intro text, after creating and saving an article in the Page builder editor, switch to the standard joomla editor, add intro text before "read more" separator (don't delete the text from SP PB editor which you will see there), then save the changes, switch back to the SP PB editor and again save the changes.
Sample screenshot for your convenience: https://prnt.sc/4Ek5qkqFBVpd
Hope this helps you.
Best Regards