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.

Step 1 

Open your Modern Communication site and click the Settings icon on the top. Choose Site Contents.

Step 2

Select Site Pages.

Step 3

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.

Step 4

Please click the New button as shown in this screenshot:

Step 5

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!