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After doing some research (there was not much on this error online), I found the issue. I was logging in as a Farm Administrator account and apparently you cannot add apps using a Farm Admin account - which seems strange that the error does not reflect that and the fact that I have permission to everything else.
I created a test non-admin account and gave the account full control to a test site, and it added the app just fine without any issues.
Let me know if this helps you!