Hi @J-3804 ,
What's the type of your account(pop,imap or exchange)?
According to my research and tests, "Always prompt for logon credentials" option is also gray on my client, but I found we could set the register key to configure it
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\Security
Type: Reg_Dword Name: PromptForCredentials value:1
But I noticed that it could work for 1 account but not for another Microsoft 365 account, so I think it may also depend on your account, it's recommended to test if it could work for you.(Note: Serious problems can occur if you modify the registry incorrectly. Before making changes, back up the registry to restore it in case something goes wrong.)
If that's invalid for you and if you just would like set it to prevent unintentional intrusion by other people who share your computer, as a workaround, it's recommended to create a password for an Outlook Data File (.pst), you would also receive a password prompt when you open the Outlook after that, for more information, please refer to this document: Set a password to help protect your Outlook information.
Hope that would be helpful to you.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.