Using older versions of Outlook for Mac for personal email and calendar
Hi,
Who is the email provider of the account?
First, make sure Outlook isn't in Offline mode. In Outlook, select Tools → Accounts, select the affected account, and verify that the account is connected and enabled.
Outlook → Settings → Accounts
Select the affected account → – to remove it.
Quit and reopen Outlook.
Add the account again and allow Outlook to synchronize the mailbox.
I look forward to hearing from you to get the results of the troubleshooting.