This article explains how to fix the error in Windows Outlook where the drop-down menu for attachments disappears when clicked, preventing users from attaching files through this method.
Overview: This glitch is easily fixed by signing out and back in to your Office 365 account following these directions

Step 1.  Sign out of your Outlook Account using the following steps:

  1. Open Outlook and click File in the top left. 
  2. Click Office Account on the left-hand sidebar.
  3. Press Sign Out under the "User Information" section.

Step 2. Close Outlook by pressing the "X" in the top-right corner of the window.

Step 3. Reboot your computer completely.

Step 4. Open the Outlook application and sign in to your account using the following steps:

  1. Open Outlook and press on the File in the top left. 
  2. Press on Office Account on the left-hand sidebar.
  3. Press Sign In under the "User Information" section.
  4. Use your WSU credentials to sign-in to Office 365.
  5. Once authentication website opens, use your WSU credentials to sign-in.