Hello Robert Allen,
To access or switch between separate Outlook.com accounts in your browser, select your profile picture or initials in the upper-right corner, then select Sign in with a different account or Sign out. Enter the other Outlook.com account’s email address and password.
If you instead mean adding or deleting a connected email account under Settings > Accounts, Microsoft has removed the ability to add new third-party accounts, such as Gmail or Yahoo, or synchronize existing connected accounts within Outlook.com. You can find more information in Microsoft’s official article: Connected email accounts like Gmail and Yahoo not working in Outlook.com.
To clarify, are you trying to add another Outlook.com account, a Gmail/Yahoo account, or a work/school mailbox? Are you using a personal Outlook.com account or an organization-managed account?