Your screen shot is the display of an already-installed add-on.
You're looking in the wrong place for install add-on, you should be at the top of the "manage add-ons" page, which has the install link at the top right. But you're also on Cloud according to your labels, so you won't have it - you can only add Cloud or the ones Atlassian allow - see https://confluence.atlassian.com/cloud/add-ons-and-plugins-744721612.html
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