Either you don't have admin rights, or you are on Cloud where you can't upload add-ons directly.
On server, yes, there are ways to upload and bypass the plugin manager, but frankly, don't bother. Get your access fixed so you can upload things properly. On Cloud, you can manage the standard add-ons via your my.Atlassian account (selecting which ones you need), or install "Connect" add-ons by browsing the marketplace (public and your private uploads)
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