Case Related List in Opportunities does not appear in Lightning Interface

Option 1: Use a Custom Lookup Field:

1. Go to Setup → Object Manager → Case → Fields & Relationships → New → Lookup Relationship → Opportunity.

2. Create a lookup field on the Case object that references Opportunity.

3. Go to Setup → Object Manager → Opportunity → Page Layouts.

4. Choose the relevant layout.

5. In the palette at the top, find Related Lists.

6. Drag Cases (or your custom lookup relationship label) into the Related Lists section.

7. Save the layout.

Option 2: Add a Related List  - Single Component on the Opportunity Lightning Record Page to call out the related list. 

1. Open an Opportunity Record
2. Click the Gear - like icon beside the notification bell and select 'Edit Page'
3. Drag the Component : Related List - Single in your record page
4. Set Parent to "Account Name"
5. Select Case for Related List drop down
6. Save and Activate your Page (if needed)