I would like you to enable safe mode and then see if you still unable to see the upload add-on? If you able to see the upload add-on button after you enable the safe mode it means one of your 3rd party plugin causing this issue. I suggest you to disable the 3rd party plugin one by one and see which plugin causing this issue.
If you are using "Cloud", then you cannot upload addons. You need to go via your my.atlassian account.
If you are running "server" version, then you need to be an admin to upload addons, but even then, you really should use the "manage plugins" section to get it from the marketplace, rather than upload manually.
Do you use templates with Confluence? Take part in a remote 1-hr workshop. You'll receive USD $100 for your time! We're looking for people to participate in a remote 1-hr workshop...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs