On Modern Communication sites, for the ShortPoint Page Builder to be available on the page you are building, please, create a new page as explained below.
Open your Modern Communication site and click the Settings icon on the top. Choose Site Contents.
Select Site Pages.
If in the bottom left corner you see the option Return to classic SharePoint, please click it. If you are already in the classic experience, please skip this step.
Please click the New button as shown in this screenshot:
Please type in a name for your page and press Create:
That's it! Your page is ready, and now you can use ShortPoint on it. To open the Page Builder, please click the Insert tab, and you will see the Insert button:
Thank you for your attention!