The user experience in Confluence is slightly different than in Reports or in Jira Dashboards and depends on the platform you use - Cloud or Serve/DC.
Cloud
How to add
Step 1. Create a page and click on “+” to add an element to the page. In the opened dialog, you can start typing the name of the gadget to find it. To add a gadget, click on it.
Step 2. Select the target chart and click on “Create chart.” FYI: You can scroll the dialog.