Generic Outlook accounts only have so much space within them. Just like filling up a glass of water, only so much can fit in there. Outlook is suggesting you upgrade so that you don't get a new e-mail address to have to give everyone, but are paying to get a fuller mailbox. Your other option would be to delete useless, outdated, or unwanted e-mails.
Your issue is also not consistent with the OP's message, so you should have started a new topic.
The only work around i found so far is it let me add my att email to Outlook app so I can see my emails there for now. It's not ideal but it works til Samsung hopefully figures this out with a future update.
