If your site has the new Confluence Cloud editor, the Panel Macro is gone. It's been replaced by a number of specific panel macros such as Info panel, Error panel, and so on. (I've attached a screenshot below of the choices available.)
There's an open request for Atlassian to Bring back the original panel macro feature if you'd like to add your vote and comments.
The rather good and detailed e-learning skillbuilder from Atlassian University called Confluence Content Management (https://training.atlassian.com/confluence-content-management) recommends using the old Panel macro. In fact, in that e-leaning course Atlassian states that the old Panel macro is key to creating ordered pages that are easy to read and maintain.
Really makes you wonder why they removed a macro they themselves believe was very important. On a side note it is also peculiar that the e-learning course has not been updated.
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