If the "Always prompt for logon credentials" option is greyed out in Outlook, it may be because your email account is configured to use modern authentication, which is a more secure authentication method that uses OAuth 2.0 tokens instead of traditional username and password authentication.
With modern authentication, Outlook doesn't store your password locally and instead uses a token to authenticate your account. This means that you won't be prompted for your password each time you open Outlook, as long as the token is valid.
If you want to disable modern authentication and enable the option to always prompt for logon credentials, you can do so by modifying a registry setting. Here's how:
Close Outlook if it's open.
Press the Windows key + R to open the Run dialog box.
Type "regedit" and press Enter to open the Registry Editor.
Navigate to the following registry key: HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common\Identity
Note: The "16.0" value in the path may vary depending on the version of Outlook you have installed. For example, if you have Outlook 2013 installed, the path may be: HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\15.0\Common\Identity
Right-click on the Identity key and select New > DWORD (32-bit) Value.
Name the new value "DisableADALatopWAMOverride" (without the quotes).
Double-click on the new value and set its value data to "1" (without the quotes).
Click OK to save the changes.
Close the Registry Editor.
After making this change, Outlook will use basic authentication instead of modern authentication, and the option to always prompt for logon credentials should now be available. Note that disabling modern authentication may make your email account less secure, so you should only do this if you need to meet a specific security requirement.