Campaign Influence related list does not appear on Opportunities page

You need to ensure that the related list is first added to the page layout. The page layout set by the administrator should either override the User's personal setting or the User's personal setting must contain the related list.

Steps to check the Personal Settings of User to see if the Related list is added:

1. Click the avatar icon and click Settings menu.
2. On the left side, click “Display and Layout”
3. Open “Customize My Pages”
4. On the dropdown, select “Opportunities”
5. From the Available List, select “Campaign Influence” and add it to the Selected list
6. Click Save.

Note:
        The profile of the User also needs to have "Read" on Contacts to display the Campaign Influence related list.

• If you don't see "Campaign Influence" in the available list, please check and make sure your Campaign Influence Setting is enabled. See Campaign Influence Related List not available on Opportunities  
• To view and edit Campaign Influence related lists, users need the Campaign Influence permission and a Sales User, Service User, or CRM User permission set license. See  Add Customizable Campaign Influence Related Lists