Missing Establish button on Opportunity Products

The Establish button is hidden when 'Quantity Scheduling Enabled' or 'Revenue Scheduling Enabled' checkboxes are absent from the Product page layout, or when the Schedules related list is missing from the Opportunity Product layout. Follow these steps to resolve both issues.

Steps to add fields to the Page Layout:

In Salesforce Classic, Go to setup -->Customize-->Products-->Page Layout-->Click Edit besides the page layout name-->Drag and Drop Quantity Scheduling Enabled and Revenue Scheduling Enabled-->Click Save
You may need to edit the Field Level Security for the fields to make them visible after adding them to the page layout.

To make the button visible:

1. Go to Setup.
2. Navigate to Customize > Opportunity > Opportunity Products.
3. Click Page Layouts.
4. Click Edit next to the name of Opportunity Product Layout.
5. On the pallet, click Related Lists.
6. Then drag and drop Schedules to the layout.
7. Click Save button.

Once done check on the checkbox in the Product as per the requirement and Establish Button should then be visible in the Opportunity Product. If you find that the button does not appear on the 'Schedules' related list, please make sure that to grant Read access to the Date Field on Opportunity Product. After these steps are taken the Establish button should be visible.