Outlook 2016 not able to attach. Attach file menu disappear immediately after click on it. 

Solution :


Close and exit Outlook. Go to Control Panel, depending on what view you have it set to. Mine is small icons (upper right corner).  

Open Credential Manager and look for 3 MicrosoftOffice16_Data...  entries under Generic Credentials. 

Open each one and "Remove from vault".   These entries will recreate next time you enter Office. 

Open Outlook and now you should be able to attach.   Hope it works !