Issue : Customize Form button is missing on SharePoint 2010 list ribbon.
Additional details :
– This site runs on SharePoint Enterprise Edition (Form services are available only in Enterprise edition)
Resolution: In this case, I had to enable Enterprise features through central administration.
Central Administration -> Upgrade and Migration ->Enable Features on existing sites
Enable Enterprise features
Once its completed, “Customize Form” button will appear on list ribbon.
Customize Form
SharePoint Exploration 