Cannot View an Attachment in Task Records

In the Task page layout, the 'Attachments' related list might have been removed via Personal Settings. To add this related list please follow one of the click paths by logging in as the Affected User:

Note: The Attachment related list can be made available in Classic only. Remember that although you can adjust your personal settings in both UIs, the Attachment related list itself cannot be made available in Lightning. For any sort of attachment, the option for Lightning Users is Files.
 

• In Salesforce Classic: Name > My Settings > Display and Layout > Customize My Pages > Select "Tasks" > Click "Customize Page" > Add "attachments" to selected list" > Save

• In Lightning Experience: Avatar > Settings > Display & Layout  > Customize my pages > select "Tasks" > Click "Customize Page" > Add "files" to selected list" > Save

If Files is unavailable, please follow these steps:

In Lightning Experience

1. Go to Setup.
2. Click Object Manager.
3. Click Task.
4. Click Page Layout.
5. From Palette, click Related List.
6. Drag Files into Related Lists Section.
7. Save.

In Salesforce Classic

1. Go to Setup.
2. Enter Task Page Layouts in the Quick Find Box.
3. Click Edit.
4. From Palette, click Related List.
5. Drag Files into Related Lists Section. 
6. Save.

Once you complete these steps, Files should now be available. 

Now the Attachments related list is available while creating tasks using the 'New Task' button or when editing an existing task record. Note the Attachment related list will not display when creating a task using the 'Log a Call' button.