Using classic Outlook for Windows in business environments
You're using new outlook, not classic outlook. If the account is not an outlook.com or Microsoft 365 business accounts, it is IMAP or POP and the display name is not editable at this time. It's going to be your email address. Sorry.
You can leave Feedback about this from the Help tab > Feedback or in the Outlook Feedback Portal at https://feedbackportal.microsoft.com/feedback/forum/89a8afa3-2e1c-ec11-b6e7-0022481f8472